drm/i915: Extract i915_module.c

The module init code is somewhat misplaced in i915_pci.c, since it
needs to pull in init/exit functions from every part of the driver and
pollutes the include list a lot.

Extract an i915_module.c file which pulls all the bits together, and
allows us to massively trim the include list of i915_pci.c.

The downside is that have to drop the error path check Jason added to
catch when we set up the pci driver too early. I think that risk is
acceptable for this pretty nice include.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727121037.2041102-11-daniel.vetter@ffwll.ch