drm/amd/display: Move panel_cntl specific register from abm to panel_cntl.

[Why]
panel_cntl specific register should be access in panel_cntl object.

[How]
Move these register access from abm to panel_cntl.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>