gbm: Don't traverse backwards for includes

This is just a bad idea and should be avoided. Instead, make the #include
flat and fix the build systems to pass the proper -I flags

v2: - add an inc_wayland_drm instead passing a path to
      include_directories (Emil)
    - update commit message (Emil)

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)