drm/amd/display: Add DPCS regs for dcn302 link encoder

dpcs reg are missing for dcn302 link encoder regs list, so add them.
Just like dcn3

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>