media: venus: Constify struct dec_bufsize_ops and enc_bufsize_ops

"struct dec_bufsize_ops and "struct enc_bufsize_ops" are not modified in
this driver.

Constifying these structures moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
   text	   data	    bss	    dec	    hex	filename
  12494	    822	      0	  13316	   3404	drivers/media/platform/qcom/venus/hfi_plat_bufs_v6.o

After:
   text	   data	    bss	    dec	    hex	filename
  12766	    566	      0	  13332	   3414	drivers/media/platform/qcom/venus/hfi_plat_bufs_v6.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>