media: s5p-mfc: Store s5p_mfc_ctx in vb2_queue.drv_priv

media: s5p-mfc: Store s5p_mfc_ctx in vb2_queue.drv_priv

The driver stores a pointer to a v4l2_fh in the vb2_queue.drv_priv,
which it then has to cast to a s5p_mfc_ctx pointer in all vb2 queue
operations. Store the pointer to the s5p_mfc_ctx instead to avoid the
cast. While at it, use the vb2_get_drv_priv() helper to access the
field.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>