This commit adds missing luminance control registers to enable a more
standard way (VESA) to deal with eDP luminance control.

Cc: Anthony Koo <anthony.koo@amd.com>
Cc: Iswara Negulendran <iswara.nagulendran@amd.com>
Cc: Felipe Clark <felipe.clark@amd.com>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230412000943.429031-1-Rodrigo.Siqueira@amd.com