drm/i915: extract ibx_irq_uninstall

Just like ibx_irq_preinstall. We'll call this from somewhere else in
the next patch.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>