drm-backend: stop parsing IN_FORMATS blobs, use libdrm instead

Before this change the drm-backend in Weston did the work of parsing DRM
blobs in order to query IN_FORMATS data, if available. This is also the
case for other DRM/KMS clients that use IN_FORMATS (i.e. X).

libdrm 2.4.108 with e641e2a6 ("xf86drm: add iterator API for DRM/KMS
IN_FORMATS blobs") introduced a dedicated API for querying IN_FORMATS data.

Bump the minimum required version to 2.4.108, stop parsing IN_FORMATS in
Weston and start using DRM iterators. In addition, remove fallback code for
libdrm <2.4.107.

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