drm/imx: ignore plane updates on disabled crtcs

This patch fixes backtraces like the following when sending SIGKILL to a
process with a currently pending plane update:

    [drm:ipu_plane_atomic_check] CRTC should be enabled
    [drm:drm_framebuffer_remove] *ERROR* failed to commit
    ------------[ cut here ]------------
    WARNING: CPU: 3 PID: 63 at drivers/gpu/drm/drm_framebuffer.c:926 drm_framebuffer_remove+0x47c/0x498
    atomic remove_fb failed with -22

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>