egl: Change the way drivers are loaded.

egl: Change the way drivers are loaded.

Driver is chosen and preloaded when eglGetDisplay is called.  Later when
eglInitialize is called, the same driver is matched to initialize the
display.  Also, add new, but unused, hooks to EGLDriver to allow a
driver to probe a display or unload itself.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>