drm: renesas: shmobile: Use drmm_universal_plane_alloc()

According to the comments for drm_universal_plane_init(), the plane
structure should not be allocated with devm_kzalloc().

Fix lifetime issues by using drmm_universal_plane_alloc() instead.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/65ebf0513e2836227a9636e922f51c668bd69720.1694767209.git.geert+renesas@glider.be