drm/i915: Remove runtime PM for SNB

Remove runtime PM support for SNB as it breaks hotplug support.
Feedback from V. Syrjala.

Signed-off-by: Carlos Santa <carlos.santa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>