drm/amd/display: Optimize cursor position updates

[why]
Updating the cursor enablement register can be a slow operation and accumulates
when high polling rate cursors cause frequent updates asynchronously to the
cursor position.

[how]
Since the cursor enable bit is cached there is no need to update the
enablement register if there is no change to it.  This removes the
read-modify-write from the cursor position programming path in HUBP and
DPP, leaving only the register writes.

Reviewed-by: Josip Pavic <josip.pavic@amd.com>
Signed-off-by: Aric Cyr <Aric.Cyr@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>