egl/dri2: Add a helper for the number of planes for a FOURCC format

This also serves as a convenient "is this a fourcc format" check as well
which we'll take advantage of in the next commit.

Cc: mesa-stable@lists.freedesktop.org
Tested-By: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit b95896f4)