drm/i915: Move drm_framebuffer_unreference out of struct_mutex for flips

intel_user_framebuffer_destroy() requires the struct_mutex for its
object bookkeeping, so this means that all calls to
drm_framebuffer_unreference must not hold that lock.

Regression from commit ab8d6675
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Mon Feb 2 15:44:15 2015 +0000

    drm/i915: Track old framebuffer instead of object

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89166


Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
[danvet: Clarify commit message slightly.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>