xf86drm: add iterator API for DRM/KMS IN_FORMATS blobs

Add support for parsing IN_FORMATS property blobs. Providing libdrm
with this functionality helps to standardise how user-space reads
kernel blobs 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>
Reviewed-by: Simon Ser <contact@emersion.fr>