drm/amd/display: Fix CAB cursor size allocation for DCN32/321

For calculating cursor size allocation, surface size was used, resulting
in over allocation

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>