Apologies for such an unspecific description, and for what almost seems like a support request for MythTV. I wouldn't post here if I were not 100% sure it must be related with the recent drm changes.
Note that the DRM APIs are intended for use by userspace components of graphics drivers / API libraries, not applications directly. MythTV shouldn't use the DRM directly for synchronization but rather use GLX synchronization APIs.
Tho not the case for MythTV on a embedded device requiring apps to use GLX wuld be to heavy. I would agree libdrm should be used.