drm/i915: Mark the overlay active only if we got ring space

After the GPU has wedged we can't turn on the overlay anymore. Only mark
it as active if we succeed in allocating ring space. This prevents a
WARN (previous;y a BUG) during driver unload if we attempted to use the
overlay after the GPU had already wedged.

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