drm/amd/display: add option to use custom backlight caps

[Why & How]
Provide option for vendors to specify a custom brightness-to-backlight
conversion profile.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>