drm/i915: enable FBC on gen9+ too

drm/i915: enable FBC on gen9+ too

Gen9+ platforms have been seeing a lot of screen flickerings and
underruns, so I never felt comfortable in enabling FBC on these
platforms since I didn't want to throw yet another feature on top of
the already complex problem. We now have code that automatically
disables FBC if we ever get an underrun, and the screen flickerings
seem to be mostly gone, so it may be a good time to try to finally
enable FBC by default on the newer platforms.

Besides, BDW FBC has been working fine over the year, which gives me a
little more confidence now.

For a little more information, please refer to commit a98ee79317b4
("drm/i915/fbc: enable FBC by default on HSW and BDW").

v2: Enable not only on SKL, but for everything new (Daniel).
v3: Rebase after the intel_sanitize_fbc_option() change.
v4: New rebase after 8 months, drop expired R-B tags.

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/1482495839-27041-1-git-send-email-paulo.r.zanoni@intel.com