drm/i915: Extract a intel_power_well_disable() function

Similar to the ->enable vfunc in patch "drm/i915: Extract a
intel_power_well_enable() function".

v2 (from Paulo):
  - Same s/i915_/intel_/ bikeshed as the previous patch.
  - Update the commit hash.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>