drm/amd/display: Fix rotated cursor offset calculation

[Why]
Underflow is observed when cursor is still enabled when the cursor
rectangle is outside the bounds of it's surface viewport.

[How]
Update parameters used to determine when cursor should be disabled.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>