drm/vmwgfx: 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
  3172    796     16    3984     f90 drivers/gpu/drm/vmwgfx/vmwgfx_buffer.o

File size After adding 'const':
  text   data    bss     dec     hex filename
  3456    512     16    3984     f90 drivers/gpu/drm/vmwgfx/vmwgfx_buffer.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>