[Why]
Current locking scheme for cursor can result in a flip missing
its vsync, deferring it for one or more vsyncs.  Result is a
potential for stuttering when cursor is moved.

[How]
Use cursor update lock so that flips are not blocked while cursor
is being programmed.

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>