[media] vpif_display: get rid of some unused vars

drivers/media/platform/davinci/vpif_display.c: In function 'vpif_channel_isr':
drivers/media/platform/davinci/vpif_display.c:363:18: warning: variable 'field'
 set but not used [-Wunused-but-set-variable]
  enum v4l2_field field;
                  ^
drivers/media/platform/davinci/vpif_display.c: In function 'vpif_calculate_offs
ets':
drivers/media/platform/davinci/vpif_display.c:505:23: warning: variable 'vpitch
' set but not used [-Wunused-but-set-variable]
  unsigned int hpitch, vpitch, sizeimage;
                       ^
drivers/media/platform/davinci/vpif_display.c: In function 'vpif_set_output':
drivers/media/platform/davinci/vpif_display.c:816:27: warning: variable 'subdev
_info' set but not used [-Wunused-but-set-variable]
  struct vpif_subdev_info *subdev_info = NULL;

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>