drm/imx: ipuv3-plane: merge ipu_plane_atomic_set_base into atomic_update

ipu_plane_atomic_set_base is called from ipu_plane_atomic_update in two
different places, depending on whether drm_atomic_crtc_needs_modeset is
true. Also depending on the same condition, this function does two
different things.
This patch removes the indirection by merging the relevant parts into
ipu_plane_atomic_update, making the actual code flow more obvious as a
result. Also remove the duplicate planar format comment, which is
already found in ipu_plane_atomic_check.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Liu Ying <gnuiyl@gmail.com>