glsl: make sure builtins are initialized before getting the shader

glsl: make sure builtins are initialized before getting the shader

The builtin function shader is part of the builtin state, released
when glReleaseShaderCompiler is called. We must ensure that the
builtins have been (re)initialized before attempting to link with the
builtin shader.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Tested-by: Rob Herring <robh@kernel.org>
Cc: mesa-stable@lists.freedesktop.org