It is often useful to figure out if a pixel_format is either YUV or RGB
especially for driver who can perform the pixel encoding conversion.
Instead of having each driver implement its own "is_this_yuv/rgb"
function based on a restricted set of pixel value, it is better to do
this in centralized manner.
We therefore add a pixel_enc member to the v4l2_format_info structure to
quickly identify the related pixel encoding.
And add helper functions to check pixel encoding.