drm/i915: Kill fbc_wm_enabled from intel_wm_config

The fbc_wm_enabled member in intel_wm_config is useless for the time
being. The original idea for it was that we'd pre-compute it and so
that the WM merging process could know whether it needs to worry
about FBC watermarks at all.

But we don't have a convenient way to pre-check for the possibility
of FBC being used. intel_update_fbc() should be split up for that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>