The spec says the register should have that value for the entire time
that FBC is enabled, so apply the WA before we enable FBC.

Notice that we also have this WA for ILK/SNB, but it is implemented at
init_clock_gating(). I could move the IVB/HSW/BDW WA code to
init_clock_gating() too, but since we recently had some complaints
about WAs not staying after being set, I'm going to play safe and keep
this here for now.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>