ipu3-imgu: Fix NULL pointer dereference in imgu_subdev_set_selection()

In the Linux kernel, the following vulnerability has been resolved:

ipu3-imgu: Fix NULL pointer dereference in imgu_subdev_set_selection()

Calling v4l2_subdev_get_try_crop() and v4l2_subdev_get_try_compose()
with a subdev state of NULL leads to a NULL pointer dereference. This
can currently happen in imgu_subdev_set_selection() when the state
passed in is NULL, as this method first gets pointers to both the "try"
and "active" states and only then decides which to use.

The same issue has been addressed for imgu_subdev_get_selection() with
commit 30d03a0de650 ("ipu3-imgu: Fix NULL pointer dereference in active
selection access"). However the issue still persists in
imgu_subdev_set_selection().

Therefore, apply a similar fix as done in the aforementioned commit to
imgu_subdev_set_selection(). To keep things a bit cleaner, introduce
helper functions for "crop" and "compose" access and use them in both
imgu_subdev_set_selection() and imgu_subdev_get_selection().