drm/i915: Move module init/exit to i915_pci.c

The module init/exit routines are a wrapper around the PCI device
init/exit, so move them across.

Note that in order to avoid exporting the driver struct, instead of
manipulating driver.features inside i915_init we instead opt to simply
exit if i915.modeset is disabled.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773227-7994-15-git-send-email-chris@chris-wilson.co.uk