[why & how]

Expose vendor specific DPCD registers for rate controlling the eDP sink
TCON's refresh rate during PSR active. When used in combination with
PSR-SU and Freesync, it is called PSR-SU Rate Contorol, or PSR-SU-RC for
short.

v2: Add all DPCD registers required
Signed-off-by: David Zhang <dingchen.zhang@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>