xf86drm: add support for populating drm formats
This change lets libdrm take care of allocation and deallocation of DRM plane formats on behalf of clients. Weston, xserver and others can use this functionaly instead of querying and parsing the drm blob.
NOTE: this change is based on weston/kms.c
Signed-off-by: Luigi Santivetti luigi.santivetti@imgtec.com