drm/omap: handle error if scale coefs are not found

drm/omap: handle error if scale coefs are not found

If get_scale_coef functions fail, they return NULL, but we never check
the return value and could do a NULL deref. This should not happen as we
ought to validate the amount of scaling already earlier, but to be safe,
add the necessary check.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502091159.7071-3-tomi.valkeinen@ti.com
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>