drm/shmem: drop VM_DONTDUMP

Not obvious why this is needed.  According to Deniel Vetter this is most
likely a historic artefact dating back to the days where drm drivers
exposed hardware registers as mmap'able gem objects, to avoid dumping
touching those registers.  shmem gem objects surely don't need that ...

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/20191016115203.20095-4-kraxel@redhat.com