virtio: Rework virtio_gpu_object_kmap()

virtio: Rework virtio_gpu_object_kmap()

Currently, virtio_gpu_object_kmap() is only called by
virtio_gpufb_create(), when a DRM framebuffer is created.

Thus, instead of returning the vmap'ed address, emit a warning
if virtio_gpu_object_kmap is called on an already mapped
object. With this change, kmap/kunmap calls are now balanced.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180925161606.17980-3-ezequiel@collabora.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>