drm/amd/display: Use correct plane for CAB cursor size allocation

[Why&How]
plane and stream variables used for cursor size allocation calculation
were stale from previous iteration. Redo the iteration to find the
correct cursor plane for the calculation.

Reviewed-by: Alvin Lee <Alvin.Lee2@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>