drm/vkms: init plane using drmm_universal_plane_alloc

By using drmm_universal_plane_alloc instead of
drm_universal_plane_init, we let the DRM infrastructure handles
resource allocation and cleanup. We can also get rid of some
code repetitions for plane cleanup, improving code maintainability
in vkms.

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/3bbdabed0274d2d0917d1b829dd16f13d7b495f5.1619250933.git.melissa.srw@gmail.com