drm/i915: Kill fbc_enable from hsw_lp_wm_results

We don't need to store the FBC WM enabled status in each watermark
level. We anyway have to reduce it down to a single boolean, so just
delay checking the FBC WM limit until we're computing the final
value.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>