Fix compiler warnings generated by older toolchains/compiler

This fixes warnings like ``may be used uninitialized''

libweston/compositor-drm.c: In function 'drm_device_is_kms':
libweston/compositor-drm.c:6374:12: warning: 'id' may be used uninitialized in this function [-Wmaybe-uninitialized]
  b->drm.id = id;

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>