video: adf: add helpers for validating custom formats

Many custom formats look a lot like the standard ones, but with
different subsampling, bpp, etc.  Expose and document
adf_buffer_validate()'s main body, so drivers can reuse its logic when
validating these formats.

Change-Id: I1d06981c9e5aab26f3ab2956c08c679f2c823bcc
Signed-off-by: Greg Hackmann <ghackmann@google.com>