rpi-default-providers.inc: use virtual/libgbm instead of libgbm
rpi-default-providers.inc: use virtual/libgbm instead of libgbm

This is to be consistent with the latest OE.

Signed-off-by: Ming Liu <070ee461bca4578ddd34d91eec07d1f455f3cdee@gmail.com>

schnitzeltony/meta-raspberrypi,leon-anavi/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi,leon-anavi/meta-raspberrypi,leon-anavi/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi