glsl: exit loop early if we find xfb layout qualifers
glsl: exit loop early if we find xfb layout qualifers

Reviewed-by: Lionel Landwerlin <9463032a5b99ffad10f36631678a833369a311b5@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler