drm/i915: Refactor power well refcount inc/dec operations

We increase/decrease the power well refcount in several places now, and
all of those places need to do the same thing, so pull that code into
a few small helper functions.

v2: Rename the funcs to __intel_power_well_{get,put}

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>