media: ti-vpe: cal: Set cal_dmaqueue.pending to NULL when no pending buffer

When a pending buffer becomes active, the cal_dmaqueue.active field is
updated, but the pending field keeps the same value until a new buffer
becomes pending. This requires handling the special case of
pending == active in different places. Simplify the code by setting the
pending field to NULL when the pending buffer becomes active. Buffers
are now simply moved from queue to pending and from pending to active.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>