drm/radeon: track whether the GPU controls the backlight (v2)

A table in the vbios tells us whether the GPU backlight controller
is used or not.  If the bit is set, the GPU backlight controller is
used; if it is not set, an off-chip backlight controller is used.

v2: store all the firmware flags, not just BL control

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