drm/i915/fbc: enable FBC by default on HSW and BDW

These platforms should be fine now.

FBC can allow very significant power savings for screen-on idle
systems, but it is worth mentioning that a lot of people won't get
significant power savings by enabling this feature because they may
have something else preventing the system from getting into the
deepest sleep states. Examples may include a hungry wifi device or a
max_performance SATA link power management policy. You can check your
PC state residencies on the powertop "Idle stats" tab. I recommend
trying to run "sudo powertop --auto-tune" and then seeing if the
residencies improve.

Oh, and in case you - the person reading this commit message - found
this commit through git bisect, please do the following:
 - Check your dmesg and see if there are error messages mentioning
   underruns around the time your problem started happening.
 - Download intel-gpu-tools, compile it, and run:
   $ sudo ./tests/kms_frontbuffer_tracking --run-subtest '*fbc-*' 2>&1 | tee fbc.txt
   Then send us the fbc.txt file, especially if you get a failure.
   This will really maximize your chances of getting the bug fixed
   quickly.
 - Try to find a reliable way to reproduce the problem, and tell us.
 - Boot with drm.debug=0xe, reproduce the problem, then send us the
   dmesg file.

v2: Don't enable by default on SKL.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455655643-2535-1-git-send-email-paulo.r.zanoni@intel.com

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