gallium: Plumb through a way to disable GLSL const lowering

For radeonsi, we will prefer the NIR pass as it'll generate better code
(some index calculation and a single load vs. a load, then index
calculation, then another load) and oftentimes NIR optimization can kick
in and make all the access indices constant.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/mesa/main/context.c

@@ -633,6 +633,8 @@ _mesa_init_constants(struct gl_constants *consts, gl_api api)
    consts->GLSLVersion = api == API_OPENGL_CORE ? 130 : 120;
    consts->GLSLVersionCompat = consts->GLSLVersion;
 
+   consts->GLSLLowerConstArrays = true;
+
    /* Assume that if GLSL 1.30+ (or GLSL ES 3.00+) is supported that
     * gl_VertexID is implemented using a native hardware register with OpenGL
     * semantics.