ANDROID: drm/virtio: implement context init: support init ioctl

This implements the context initialization ioctl.  A list of params
is passed in by userspace, and kernel driver validates them.  The
only currently supported param is VIRTGPU_CONTEXT_PARAM_CAPSET_ID.

If the context has already been initialized, -EEXIST is returned.
This happens after Linux userspace does dumb_create + followed by
opening the Mesa virgl driver with the same virtgpu instance.

However, for most applications, 3D contexts will be explicitly
initialized when the feature is available.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>

BUG: 173630595
TEST: boot VM

Change-Id: I05690eb47efc107010b137ab6a165afadabc8366