drm/i915: Make sure we invalidate frontbuffer on fbcon.

There are some cases like suspend/resume or dpms off/on sequences
that can flush frontbuffer bits. In these cases features that relies
on frontbuffer tracking can start working and user can stop getting
screen updates on fbcon having impression the system is frozen.

So, let's make sure we also invalidate frontbuffer on fbdev blank.

v2: Daniel was right, backtrace didn't show other path than this blank
one so let's make sure frontbuffer bits gets invalidate here instead of
on random write operations that doesn't garantee we track all frontbuffer
writes.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[danvet: Exchange code comments for one that complains about the
locking, like in set_par.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>