mesa: split extensions overrides and glGetString(GL_EXTENSIONS)

mesa: split extensions overrides and glGetString(GL_EXTENSIONS)

Currently we apply the extension overrides and construct the extensions
string upon MakeCurrent.

They are two distinct things, so let's slit the two while pushing the
overrides management _before_ _mesa_compute_version(). This ensures that
the version is updated to reflect the enabled/disabled extensions.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>