drm/virtio: fix mmap page attributes

virtio-gpu uses cached mappings, set
drm_gem_shmem_object.map_cached accordingly.

Cc: stable@vger.kernel.org
Fixes: c66df701 ("drm/virtio: switch from ttm to gem shmem helpers")
Reported-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reported-by: Guillaume Gardet <Guillaume.Gardet@arm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Guillaume Gardet <Guillaume.Gardet@arm.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200226154752.24328-3-kraxel@redhat.com