xf86drm: add iterator API for DRM/KMS IN_FORMATS blobs

Add support for parsing and traversing IN_FORMATS property blobs. Providing libdrm with this functionality improves consistency and decreases duplication on the client side.

drmModeFormatModifierBlobIterNext() allows the caller to view formats and associated modifiers given a valid property blob. An example is available inside the libdrm unit test, modetest.c.

Signed-off-by: Luigi Santivetti luigi.santivetti@imgtec.com