drm/amd/display: Fix incorrect DPCD configs while Replay/PSR switch

[Why]
When switching between PSR/Replay,
the DPCD config of previous mode is not cleared,
resulting in unexpected behavior in TCON.

[How]
Initialize the DPCD in setup function

Reviewed-by: Robin Chen <robin.chen@amd.com>
Signed-off-by: Leon Huang <Leon.Huang1@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>