drm: add PSR2 support and capability definition as per eDP 1.5

[why & how]
As per eDP 1.5 spec, add the below two DPCD bit fields for PSR-SU
support and capability:

1. DP_PSR2_WITH_Y_COORD_ET_SUPPORTED
2. DP_PSR2_SU_AUX_FRAME_SYNC_NOT_NEEDED

changes in v2
------------------
* fixed the typo
* explicitly list what DPCD bit fields are added

Signed-off-by: David Zhang <dingchen.zhang@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>