drm/radeon: add backlight control for atom devices (v2)

On systems that use the build in GPU backlight controller,
we can use atom tables to change the brightness level.

v2: use firmware flags

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

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

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

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