drm/omap: fix plane check when crtc is disabled

I sometimes see:

[drm:drm_framebuffer_remove [drm]] *ERROR* failed to reset crtc ed2a6c00
when fb was deleted: -22

which comes from drm_framebuffer_remove() when it's disabling the crtc
with zeroed drm_mode_set.

The problem in omap_plane_atomic_check() is that it will use those
zeroed fields to verify if the setup is correct.

This patch makes omap_plane_atomic_check() return 0 if the crtc is
disabled.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>