[Why]

PSR-SU Rate Control - or PSR-SU-RC - enables PSR-SU panels to work with
variable refresh rate to allow for more power savings. Lowering the
refresh rate can increase PSR residency by expanding the eDP main link
shut down duration. It can also lower panel power consumption.

There is a complication with PSR, since the eDP main link can be shut
down. Therefore, the timing controller (TCON) on the eDP sink nees to be
able to scan out its remote buffer independent of the main link. To
allow the eDP source to specify the sink's refresh rate while the link
is off, vendor-specific DPCD registers are used. This allows the eDP
source to then "Rate Control" the panel during PSR active.

[How]

Add DC support to communicate with PSR-SU-RC supported eDP sinks. The
sink will need to know the desired VTotal during PSR active.

This change only adds support to DC, support in amdgpu_dm is still
pending to enable this fully.
Signed-off-by: David Zhang <dingchen.zhang@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>