drm/omap: 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
   1240	    112	      0	   1352	    548 drivers/gpu/drm/omapdrm/omap_gem_dmabuf.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1352	      0	      0	   1352	    548 drivers/gpu/drm/omapdrm/omap_gem_dmabuf.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>