drm/amd/display: Keep linebuffer pixel depth at 30bpp for DCE-11.0.

Testing on AMD Carizzo with DCE-11.0 display engine showed that
it doesn't like a 36 bpp linebuffer very much. The display just
showed a solid green.

Testing on RavenRidge DCN-1.0, Polaris11 with DCE-11.2 and Kabini
with DCE-8.3 did not expose any problems, so for now only revert
to 30 bpp linebuffer depth on asics with DCE-11.0 display engine.

Fixes: a316db72

 ("drm/amd/display: Increase linebuffer pixel depth to 36bpp.")
Reported-by: Tom StDenis <Tom.StDenis@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>