drm/shmem: drop VM_IO

VM_IO is wrong here, shmem uses normal ram not io memory.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191016115203.20095-5-kraxel@redhat.com