Add a helper to reduce a pixel format to a "base" format.
Ie. group all formats to some idea of classes based on the
bits per pixel + component sizes etc. ignoring any component ordering.
Let's us skip potentially redundant tests when all we might care
about is testing each format "class" the once.

v2: Add some docs (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #irc
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>