drm/omap: print error instead of WARN() if plane setup fails

omap_plane_atomic_update() does WARN_ON() if dispc rejects the given
plane config. Change that to dev_err() to lessen the possible spam.

To fix this correctly, the plane setup needs much more work by creating
a check function for dispc setup, so that we could reliably check the
config in atomic_check, instead of only noticing the problem when
programming dispc.

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