drm/amd/display: use actual cursor size instead of max for CAB allocation

[Why&How]
When calculating allocation for cursor size, get the real cursor through
the HUBP instead of using the maximum cursor size for more optimal
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>