drm/radeon: add a connector property for dither

Allows you to enable dither in the display hardware
when the monitor supports lower a lower bpc than the
current framebuffer format.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/cik.c
Simple merge

diff --cc drivers/gpu/drm/radeon/evergreen.c
Simple merge

diff --cc drivers/gpu/drm/radeon/r600.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_connectors.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_display.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_mode.h
Simple merge

diff --cc drivers/gpu/drm/radeon/rs600.c
Simple merge