drm/i915: don't enable FBC when pixel rate exceeds 95% on HSW/BDW
BSpec says we shouldn't enable FBC on HSW/BDW when the pipe pixel rate
exceeds 95% of the core display clock.
v2:
- HSW also needs the WA (Ville).
- Add the WA name (Ville).
- Use the current cdclk (Ville).
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>