drm/i915: Disable FBC WM on ILK, and disable LP2+ when FBC is enabled

ILK has a bunch of issues with FBC. First of all, BSpec tells us that
FBC WM should never be enabled. Secondly when FBC is enabled
with FBC WM disabled, LP2+ watermarks must be disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>