st/dri: use local pointer to st_context_iface
st/dri: use local pointer to st_context_iface

Reviewed-by: Timothy Arceri <c48760dafc316d20cf5b0817150c999e82cd892f@itsqueeze.com>
Reviewed-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler