drm/amd/display: Refactor PSR DPCD caps detection

[Why]
To move the PSR DPCD caps detection into detect_edp_sink_caps()

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Po Ting Chen <robin.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>