drm/amd/display: Update Cursor request mode to the beginning prefetch always

[Why]
The double buffer cursor registers is updated by the cursor
vupdate event. There is a gap between vupdate and cursor data
fetch if cursor fetch data reletive to cursor position.
Cursor corruption will happen if we update the cursor surface
in this gap.

[How]
Modify the cursor request mode to the beginning prefetch always
and avoid wraparound calculation issues.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Zhikai Zhai <zhikai.zhai@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>