drm/i915: Fix module unloading with DRM_I915_UMS=n

Oops, makes testing early boot failures in i915.ko a bit more pain, so
let's fix it.

v2: We already have a bit of static storage to track this (Chris).

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>