egl: Make egl+drm independent at link time of the Mesa GBM
This adds a dlsym based function table for gbm functions, so we only have a runtime dependency. That way we can build egl with drm support without building gbm. (ChromeOS ahs that for reasons ...)
Also does the dladdr trick for checking if a display is a gbm display.
Edited by Bas Nieuwenhuizen