drm/i915: Split out load time interface registration

According to the new init phases scheme we should register the device
making it available via some kernel internal or user space interface as
the last step in the init sequence, so move the corresponding code to a
separate function.

Also add a TODO comment about code that still needs to be moved around
to one of the init phases functions depending on what the role and effect
of that code is.

No functional change, except for the reordering of the unload time
unregistration steps of sysfs wrt. acpi and opregion.

Suggested by Chris.

v3:
- rename i915_driver_init_register to i915_driver_init_frameworks
  (Chris)
- rename i915_driver_init_frameworks to i915_driver_register (Daniel)

CC: Chris Wilson <chris@chris-wilson.co.uk>
CC: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-18-git-send-email-imre.deak@intel.com