WIP: drm: Introduce RVKMS!

Now that we've added all of the bits that we need for the KMS API, it's
time to introduce rvkms! This is a port of the VKMS driver to rust, with
the intent of acting as an example usecase of the KMS bindings that we've
come up with so far in preparation for writing a display driver for nova.

Currently RVKMS is an extremely bear bones driver - it only registers a
device and emulates vblanking, but it exercises a good portion of the API
that we've introduced so far! Eventually I hope to introduce CRC generation
and maybe writeback connectors like.

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