This commit adds bindings for implementing vblank support for a driver's
CRTCs. These bindings are optional, to account for the fact that not all
drivers have dedicated hardware vblanks.

In order to accomplish this, we introduce the VblankSupport trait which can
be implemented on DriverCrtc by drivers which support vblanks. This works
in the same way as the main Kms trait - drivers which don't support
hardware vblanks can simply pass PhantomData<Self> to the associated type
on DriverCrtc. If a driver chooses to implement VblankSupport, VblankImpl
will be implemented by DRM automatically - and can be passed to the
VblankImpl associated type on DriverCrtc.

Additionally, we gate methods which only apply to vblank-supporting drivers
by introducing a VblankDriverCrtc trait that is automatically implemented
by DRM for CRTC drivers implementing VblankSupport. This works basically in
the same way as Kms and KmsDriver, but for CRTCs.

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

---

Notes:

* One thing to keep in mind: this trait is implemented on the CRTC as
  opposed to the KMS driver due to the possibility that a driver may have
  multiple different types of CRTCs. As a result, it's not impossible that
  there could potentially be differences in each type's vblank hardware
  implementation. In theory this could lead to a driver mistakenly only
  implementing VblankSupport for some CRTCs and not others, which isn't
  really defined behavior in DRM. As such, one of the dependencies in the
  branch for this patch series preview is a fix to ensure that DRM
  disallows registering drivers that make this mistake.

TODO:

* Technically this patch introduces a soundness issue. We currently allow
  access to a kernel::drm::Device (through UnregisteredKmsDevice's Deref
  implementation) within the kernel::drm::kms::Kms::create_objects trait
  method. A caller could potentially access an uninitialized mutex by
  calling Crtc::vblank_lock() within this context.
  My solution for this is likely going to be adding unregistered variants
  of Crtc and other mode-objects that don't have access to the full set of
  methods on mode objects.