drm/udl: dma-buf: Constify dma_buf_ops structures.

drm/udl: dma-buf: Constify dma_buf_ops structures.

dma_buf_ops are not supposed to change at runtime. All functions
working with dma_buf_ops provided by <linux/dma-buf.h> work with
const dma_buf_ops. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   2002	    112	      0	   2114	    842	drivers/gpu/drm/udl/udl_dmabuf.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2114	      0	      0	   2114	    842	drivers/gpu/drm/udl/udl_dmabuf.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/cb4dfac90e85e2270779331f8cb10b635042bad7.1498912415.git.arvind.yadav.cs@gmail.com