drm/amd/display: add debug keys for override bios settings.

[why]
adding debug keys used for compliance test.

Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>