drm/i915: group all hotplug related fields into a new struct in dev_priv

There are plenty of hotplug related fields in struct drm_i915_private
scattered all around. Group them under one hotplug struct. Clean up
naming while at it. No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>