media: vivid: Use v4l2_find_nearest_size

Use v4l2_find_nearest_size instead of a driver specific function to find
nearest matching size.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>