drm/i915: unbind fbs from crtcs during driver unload

So far during driver unload we called drm_framebuffer_cleanup() for the
fbdev fb, which only removes the fb from the drm fb list regardless of
its reference count, but leaves the fb bound on an active crtc. Since
the fb's backing storage was freed this could mean we scan some random
memory content out afterwards. It's not a big issue since the fb is
allocated from stolen memory and afaik there is no other user for that
than i915. It's still cleaner to properly unbind the fb and disable the
crtc, which is what drm_framebuffer_remove() does.

Note that after

commit 88891eb1e9eca0ba619518bed31580f91e9cf84d
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Feb 10 18:00:38 2014 +0100

we call drm_framebuffer_cleanup() only after dropping the last reference
on the fb, but that won't happen since we don't unbind the fb. This
results in a drm core warn about a leaked fb.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>