With 4.15 (and the kernel boot parameter!) I now managed to get audio output
via HDMI. 

Splendid!

My configuration is 

PC ----HDMI---> Pioneer AV receiver (vsx330) ---HDMI---> HP monitor (lp2475w)

But there is one bad effect now: When I turn off the monitor during runtime, or
if the monitor enters power-save state, and it is turned on again, the screen
resolution is reset to a maximum of 1368x768 (before: 1920x1200), which messes
up the desktop.

The reason is that when the monitor is turned off, this seems to be sensed as a
change in the HMDI connection, and the EDID data is now coming from the AV
receiver. 

dmesg says:

[   82.689994] [drm] VSX-330: [Block 0] 
[   82.689995] [drm] VSX-330: [Block 1] 
[   82.689997] [drm] dc_link_detect: manufacturer_id = 2F41, product_id = 1027,
serial_number = 1010101, manufacture_week = 0, manufacture_year = 25,
display_name = VSX-330, speaker_flag = 79, audio_mode_count = 8
[   82.689998] [drm] dc_link_detect: mode number = 0, format_code = 1,
channel_count = 1, sample_rate = 127, sample_size = 7
[   82.689999] [drm] dc_link_detect: mode number = 1, format_code = 1,
channel_count = 7, sample_rate = 127, sample_size = 7
[   82.690000] [drm] dc_link_detect: mode number = 2, format_code = 2,
channel_count = 5, sample_rate = 7, sample_size = 80
[   82.690000] [drm] dc_link_detect: mode number = 3, format_code = 7,
channel_count = 6, sample_rate = 6, sample_size = 192
[   82.690001] [drm] dc_link_detect: mode number = 4, format_code = 9,
channel_count = 1, sample_rate = 127, sample_size = 0
[   82.690002] [drm] dc_link_detect: mode number = 5, format_code = 10,
channel_count = 7, sample_rate = 6, sample_size = 0
[   82.690002] [drm] dc_link_detect: mode number = 6, format_code = 11,
channel_count = 7, sample_rate = 126, sample_size = 1
[   82.690003] [drm] dc_link_detect: mode number = 7, format_code = 12,
channel_count = 7, sample_rate = 126, sample_size = 0
[   82.691002] Raw EDID:
[   82.691003]          c8 01 00 00 06 00 00 00 20 00 00 00 02 00 00 00
[   82.691003]          80 07 00 00 b0 04 00 00 01 00 0a 00 fc 05 e0 01
[   82.691004]          e0 04 00 00 00 00 00 00 01 00 01 00 00 00 02 00
[   82.691004]          00 00 00 00 00 0a 00 00 01 00 00 00 70 00 00 00
[   82.691004]          08 00 02 00 fc 05 e0 01 12 00 07 00 fc 05 e0 01
[   82.691005]          9b 01 00 00 04 00 00 00 20 00 00 00 01 00 00 00
[   82.691005]          04 00 00 00 01 00 0a 00 fd 05 e0 01 e0 04 00 00
[   82.691006]          00 00 00 00 1e 00 1e 00 00 00 02 00 00 00 00 00
[   82.691007] amdgpu 0000:01:00.0: HDMI-A-2: EDID invalid.

Note that the monitor is typically labeled as HDMI-A-1.

The screen configuration in KDE lists 1368x768 as the maximum resolution now. 

I managed to restore the old resolution either by rebooting, or by turning off
the AV receiver, then turning it back on. Still, the desktop is messed up.