drm/i915: Add intel_display_power_{get, put} to request power for specific domains

Add APIs to get/put power well references for specific purposes.

v2: Split the i915_request change to another patch

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>