Suspend works again with my HD 5450 using branch 'drm-next-3.18' of 
git://people.freedesktop.org/~airlied/linux!

Previously resume consistently produced this output with a subsequent crash of
the X-server:

[ 7025.164024] radeon 0000:01:00.0: ring 5 stalled for more than 10000msec
[ 7025.164030] radeon 0000:01:00.0: GPU lockup (waiting for 0x000000000000000a
last fence id 0x0000000000000002 on ring 5)
[ 7025.164034] [drm:uvd_v1_0_ib_test] *ERROR* radeon: fence wait failed (-35).
[ 7025.164040] [drm:radeon_ib_ring_tests] *ERROR* radeon: failed testing IB on
ring 5 (-35).

Thanks for finally resolving this problem!