drm/virtio: Don't create a context with default param if context_init is supported

Xorg context creation fails for native contexts that use
VIRTGPU_CONTEXT_INIT because context is already initialized implicitly
when dumb buffer is created. Fix it by not creating default vrend context
if context_init is supported.

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241020224725.179937-1-dmitry.osipenko@collabora.com