include: Introduce linux-uapi for non-drm-uapi files.

sync_file.h is needed for dma-buf, but it is part of the
linux-uapi include dir.

The patch aims to have a drm-uapi directory that is a pure
copy of the generated drm-uapi include dir.

Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>