drm/i915: Limit FBC flush to post batch flush

Don't issue the FBC nuke/cache clean command when invalidate_domains!=0.
That would indicate that we're not being called for the post-batch
flush.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>