drm: ttm: virtio-gpu: dma-buf: Constify ttm_place structures.

drm: ttm: virtio-gpu: dma-buf: Constify ttm_place structures.

ttm_place are not supposed to change at runtime. All functions
working with ttm_place provided by <drm/ttm/ttm_placement.h> work
with const ttm_place. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   2315	    184	      0	   2499	    9c3 drivers/gpu/drm/virtio/virtgpu_ttm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2347	    152	      0	   2499	    9c3 drivers/gpu/drm/virtio/virtgpu_ttm.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/25a189402a516a0142d9a4412da0a597c660a96a.1498981093.git.arvind.yadav.cs@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>