drm/amd/display: Implement PCON regulated autonomous mode handling

[Why/How]
DP spec has been updated recently to make regulated autonomous mode more
well-defined. In case any PCON vendors choose to implement regulated
autonomous mode in the future, pre-emptively add handling for the
regulated autonomous mode based on current spec.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>