I've tested tahiti-fix.patch for kernel with version 3.16-3, it does not help.
I've added debug info to it:

radeonsi cgts_tcc_disable: -268435456

that's the value of register for my card.

patch v5 also doesn't resolve problem, after adding additional prinf's it seems
that si_init_config goes into:

if (rb_mask && util_bitcount(rb_mask) >= num_rb) {

so si_write_harvested_raster_configs is not called

after commenting out that if, xorg logs from si_write_harvested_raster_configs
shows:

Original raster_config = 0x2a00126a, rb_mask = 0xff

attachments:
- startx-0410-2.out - output of startx with -verbose 9, tahiti-fix kernel,
mesa-git (c74be01e80fcdd7feabc0f27df4aebe66abb626e) with patchv5 + additional
fprintf's
- kernel-0410-2.out - dmesg, additional debug in tahiti-fix: radeonsi
cgts_tcc_disable