drm/i915: Reject changes of fb base when we have a flip pending

This should be impossible due to the wait for outstanding flips that the
caller is meant to perform prior to updating the scanout base. Paranoia
tells me to check anyway.

References: https://bugs.freedesktop.org/show_bug.cgi?id=75502


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>