drm/i915/fbc: Enable fbc by default on glk+ once again

drm/i915/fbc: Enable fbc by default on glk+ once again

Now that we have the glk+ w/a for back to back fbc disable + plane
update in place we can once more enable fbc on glk+ by default.

Cc: Daniel Drake <drake@endlessm.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jian-Hong Pan <jian-hong@endlessm.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-14-ville.syrjala@linux.intel.com
Tested-by: Daniel Drake <drake@endlessm.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>