drm/virtio: factor out context create hypercall

We currently create an OpenGL context when opening the DRM fd
if 3D is available.

We may need other context types (VK,..) in the future, and the plan
is to have explicit initialization for that.

For explicit initialization to work, we need to factor out
virtio_gpu_create_context from driver initialization.

v2: Move context handle initialization too (olv)
v6: Remove redundant 3D check (emil.velikov)

Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200225000800.2966-2-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>