This commit adds some traits for registering DRM devices with KMS support,
implemented through the kernel::drm::kms::Kms trait. Devices which don't
have KMS support can simply use PhantomData<Self>.

Signed-off-by: Lyude Paul <lyude@redhat.com>

---

TODO:
* Generate feature flags automatically, these shouldn't need to be
  specified by the user
* Fix typo on KmsImpl documentation

V3:
* Get rid of Kms, long live KmsDriver
  After Daniel pointed out that we should just make KmsDriver a supertrait
  of Driver, it immediately occurred to me that there's no actual need for
  Kms to be a separate trait at all. So, drop Kms entirely and move its
  requirements over to KmsDriver.
* Drop fbdev module entirely and move setup_fbdev() into AllocImpl (Daniel)