MLK-22584 drm/imx: dpu: crtc: Correct plane's ExtDst source in ->atomic_begin()

MLK-22584 drm/imx: dpu: crtc: Correct plane's ExtDst source in ->atomic_begin()

In dpu_helper_funcs->atomic_begin(), we temporarily set the plane's
ExtDst source to ConstFrame in shadow.  In the plane driver,
dpu_plane_helper_funcs->atomic_update() can update the shadow if
necessary.  This way, we may set the source as either ConstFrame
or LayerBlend.  We only set the source for the old top plane in
->atomic_begin().  However, in cases where pixel combiner is used,
the top planes are tracked separately for the master stream and
the slave stream with ->is_left_top or ->is_right_top flags
instead of ->is_top flag when pixel combiner is not used.  This
patch corrects the source for the cases where pixel combiner is
used.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
(cherry picked from commit ff19118af177bab2dc765b242cf5b4392910a5e7)