drm/virtio: Add capset definitions to UAPI

Since the context-type additions to the virtio-gpu spec, these have been
defined locally in guest user-space, and virtio-gpu backend library code.

Now, these capsets have been stabilized, and should be defined in a
common space, in both the virtio_gpu header, and alongside the virtgpu_drm
interface that they apply to.

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Aaron Ruby <aruby@qnx.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
[dmitry.osipenko@collabora.com: edit commit title]
Link: https://patchwork.freedesktop.org/patch/msgid/YT3PR01MB5857E808EDF6949F2DF517FDAFA12@YT3PR01MB5857.CANPRD01.PROD.OUTLOOK.COM