[media] media: davinci: vpbe: fix build warning

recent patch with commit id 4f996594
which makes vidioc_s_crop const, was causing a following build warning,
vpbe_display.c: In function 'vpbe_display_s_crop':
vpbe_display.c:640: warning: initialization discards qualifiers from pointer target type
This patch fixes the above build warning.

Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>