drm: renesas: shmobile: Unify plane allocation

Unify primary and overlay plane allocation:
  - Enhance shmob_drm_plane_create() so it can be used to create the
    primary plane, too,
  - Move overlay plane creation next to primary plane creation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/adbc5febc0099fd1910f32a7af1c8e0f570f74b4.1694767209.git.geert+renesas@glider.be