mesa: use implementation specified MAX_VERTEX_ATTRIBS rather than hardcoded value

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index 8d4b40e..b7a783c 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -3070,12 +3070,8 @@
       }
    }
 
-   /* FINISHME: The value of the max_attribute_index parameter is
-    * FINISHME: implementation dependent based on the value of
-    * FINISHME: GL_MAX_VERTEX_ATTRIBS.  GL_MAX_VERTEX_ATTRIBS must be
-    * FINISHME: at least 16, so hardcode 16 for now.
-    */
-   if (!assign_attribute_or_color_locations(prog, MESA_SHADER_VERTEX, 16)) {
+   if (!assign_attribute_or_color_locations(prog, MESA_SHADER_VERTEX,
+                                            ctx->Const.Program[MESA_SHADER_VERTEX].MaxAttribs)) {
       goto done;
    }