drm/i915: Add intel_hpd_irq_uninstall()

Add intel_hpd_irq_uninstall() which will cancel the hotplug re-enable
timer.

Also s/i915_reenable_hotplug_timer_func/intel_hpd_irq_reenable/

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>