drm/amd/display: Increase linebuffer pixel depth to 36bpp.

Testing with the photometer shows that at least Raven Ridge DCN-1.0
does not achieve more than 10 bpc effective output precision with a
16 bpc unorm surface of type SURFACE_PIXEL_FORMAT_GRPH_ABGR16161616,
unless linebuffer depth is increased from LB_PIXEL_DEPTH_30BPP to
LB_PIXEL_DEPTH_36BPP. Otherwise precision gets truncated somewhere
to 10 bpc effective depth.

Strangely this increase was not needed on Polaris11 DCE-11.2 during
testing to get 12 bpc effective precision. It also is not needed for
fp16 framebuffers.

Tested on DCN-1.0 and DCE-11.2.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>