media: vb2 dma-contig: Constify dma_buf_ops structures

media: vb2 dma-contig: 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
   6035	    272	      0	   6307	   18a3 drivers/media/v4l2-core/videobuf2-dma-contig.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6155	    160	      0	   6315	   18ab drivers/media/v4l2-core/videobuf2-dma-contig.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>