drm/i915: Rename intel_power_domains_fini() to intel_power_domains_fini_hw()

intel_power_domains_fini() rolls back what was done in
intel_power_domains_init_hw(), so rename and move it accordingly. This
allows us adding a cleanup function later for intel_power_domains_init()
in a cleaner way.

No functional change.

v2:
- Fix checkpatch error adding missing param name to function
  declaration. (Paulo)

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806095843.13294-3-imre.deak@intel.com