drm/amd/display: Add max bpc debugfs

[Why]
Useful for testing when setting a max bpc value higher than the default
is required

[How]
Allow for reading/writing of the max_requested_bpc property of the
connector

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Mikita Lipski <Mikita.Lipski@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>