drm/i915: Factor out intel_power_well_get/put

These helpers will be needed by the next patch, so factor them out.

No functional change.

v2:
- Move the refcount==0 WARN to the new put helper. (Ville)

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>