i965: Add INTEL_DEBUG=glsl_force to force brw_wm_glsl.c.

I keep finding the desire to force this path to debug it instead of
cooking up goofy-looking testcases to do so.

src/mesa/drivers/dri/intel/intel_context.c

@@ -444,6 +444,7 @@ static const struct dri_debug_control debug_control[] = {
    { "sing",  DEBUG_SINGLE_THREAD },
    { "thre",  DEBUG_SINGLE_THREAD },
    { "wm",    DEBUG_WM },
+   { "glsl_force", DEBUG_GLSL_FORCE },
    { "urb",   DEBUG_URB },
    { "vs",    DEBUG_VS },
    { NULL,    0 }