drm/i915/fbc: Disable on HSW by default for now

>From https://bugs.freedesktop.org/show_bug.cgi?id=96461 :

This was kind of a difficult bug to track down. If you're using a
Haswell system running GNOME and you have fbc completely enabled and
working, playing videos can result in video artifacts. Steps to
reproduce:

- Run GNOME
- Ensure FBC is enabled and active
- Download a movie, I used the ogg version of Big Buck Bunny for this
- Run `gst-launch-1.0 filesrc location='some_movie.ogg' ! decodebin !
  glimagesink` in a terminal
- Watch for about over a minute, you'll see small horizontal lines go
  down the screen.

For the time being, disable FBC for Haswell by default.

Stefan Richter reported kernel freezes (no video artifacts) when fbc
is on.  (E3-1245 v3 with HD P4600; openbox and some KDE and LXDE
applications, thread begins at https://lkml.org/lkml/2016/4/26/813).
We also got reports from Steven Honeyman on openbox+roxterm.

v2 (From Paulo):
  - Add extra information to the commit message
  - Add Fixes tag
  - Rebase

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96461
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96464


Fixes: a98ee793 ("drm/i915/fbc: enable FBC by default on HSW and BDW")
Cc: stable@vger.kernel.org
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Lyude <cpaul@redhat.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465487895-7401-1-git-send-email-cpaul@redhat.com