drm/amd/display: Drop unused registers

[Why & How]
Some registers are never used in the driver
but defined. Remove them.

Reviewed-by: Roman Li <roman.li@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>