drm/i915: Track old framebuffer instead of object

Daniel Vetter spotted a bug while reviewing some of my refactoring in this
are of the code. I'll quote:

"""
> @@ -9764,6 +9768,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
>  	work->event = event;
>  	work->crtc = crtc;
>  	work->old_fb_obj = intel_fb_obj(old_fb);
> +	work->old_tiling_mode = to_intel_framebuffer(old_fb)->tiling_mode;

Hm, that's actually an interesting bugfix - currently userspace could be
sneaky and destroy the old fb immediately after the flip completes and the
change the tiling of the underlying object before the unpin work had a
chance to run (needs some fudgin with rt prios to starve workers to make
this work though).

Imo the right fix is to hold a reference onto the fb and not the
underlying gem object. With that tiling is guaranteed not to change.
"""

This patch tries to implement the above proposed change.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>