DRM/i915: Get rid if the 'hotplug_supported_mask' in struct drm_i915_private.

Now since we have replaced the bits to show interest in hotplug IRQs
we can go and nuke the 'hotplug_supported_mask'.

Signed-off-by: Egbert Eich <eich@suse.de>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>