drm/amd/display: Detect dynamic backlight support in eDP sink

[Why]
Check if the eDP sink supports the dynamic backlight control

[How]
Query the dynamic backlight capability in DPCD (eDP case only)

Signed-off-by: Roy Chan <roy.chan@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>