drm/i915/opregion: Rename init/fini functions to register/unregister

Current intel_opregion_init is called during the driver registration
phase and intel_opregion_fini from the unregistration phase. Rename the
functions so that this is clear from their names. The phases tell us
what we expect the existing hw state to be, e.g. whether interrupts are
still enabled etc.

It should be noted that the opregion init/fini routines are asymmetric
and this is carried across into their new names. Indeed, their new names
make it even clearer that perhaps all is not well in the opregion
suspend/resume sequence (as well in the module unload).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464012490-30961-2-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>