egl: simplify client/platform extension handling

egl: simplify client/platform extension handling

For GLVND reasons the client/platform extensions strings should be
split. While in the non GLVND case they're one big string.

Currently we handle this distinction at run-time for not obvious reason.
Adding additional code and complexity.

Swap those with a few well placed #if USE_LIBGLVND guards.

As a side result this removes a minor memory leak due to the
concatenation in the non GLVND case.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4491>