st/glsl_to_nir: fix next_stage gathering

ffs() just returns the bit that is set, we need to know what
stage that bit represents so use u_bit_scan() instead.

Fixes: 2ca5d9548f ("st/glsl_to_nir: gather next_stage in shader_info")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -373,7 +373,7 @@ st_glsl_to_nir(struct st_context *st, struct gl_program *prog,
          ~prev_stages & shader_program->data->linked_stages;
 
       nir->info.next_stage = stages_mask ?
-         (gl_shader_stage) ffs(stages_mask) : MESA_SHADER_FRAGMENT;
+         (gl_shader_stage) u_bit_scan(&stages_mask) : MESA_SHADER_FRAGMENT;
    } else {
       nir->info.next_stage = MESA_SHADER_FRAGMENT;
    }