drm/meson: Fix plane atomic check when no crtc for the plane

When no CRTC is associated with the plane, the meson_plane_atomic_check()
call breaks the kernel with an Oops.

Fixes: bbbe775e ("drm: Add support for Amlogic Meson Graphic Controller")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>