drm/virtio: track whether or not a context has been initiated

Use an boolean variable to track whether a context has been
initiated.

v5: Fix possible race and sleep via mutex (olv)

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/20200225000800.2966-3-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>