BACKPORT: 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>
(cherry picked from commit a20c4173c4219d85ba7775fde584cd34305a0b3d)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I970b24d86c43017aa9d58b6cdc6e4909c3870c31