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 <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Benoit Parrot <[email protected]>
Reviewed-by: Peter Ujfalusi <[email protected]>
Signed-off-by: Sean Paul <[email protected]>