drm/amd/display: Add backlight support via AUX

Several modern displays support HDR/OLED panel, and one of the
interesting changes provided by this technology is the new approach for
handling backlight. More precisely, in the traditional way, we use PWM
to manage display backlight; however, with HDR/OLED we begin to use DP
Aux channel for adjusting the backlight.

This patch, enable the backlight control in HDR/OLED display via DP Aux.

Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>