panfrost: Check for NULL surface in places

panfrost: Check for NULL surface in places

Fixes a bunch of NULL dereferences, although it does cause GPU faults of
course.

This is caused by color buffers masked out in MRT, which we'll
eventually have to solve the right way... one thing at a time.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>