egl: const _eglDriver

Converted using `s/_EGLDriver/const _EGLDriver/g` and dropped a couple
of irrelevant changes in comments, in the `_EGL_DRIVER_TYPECAST()` macro
and the typedef itself.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6129>