radv: fix dual source blending

Add the index to the location when assigning driver locations for
output variables.

Otherwise two fragment shader outputs declared as:

   layout (location = 0, index = 0) out vec4 output1;
   layout (location = 0, index = 1) out vec4 output2;

will end up aliasing one another.

Note that this patch will make the second output variable in the above
example alias a possible third output variable with location = 1 and
index = 0. But this shouldn't be a problem in practice since only one
color attachment is supported when dual-source blending is used.

Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/amd/common/ac_nir_to_llvm.c

@@ -4125,7 +4125,7 @@ static void
 handle_shader_output_decl(struct nir_to_llvm_context *ctx,
 			  struct nir_variable *variable)
 {
-	int idx = variable->data.location;
+	int idx = variable->data.location + variable->data.index;
 	unsigned attrib_count = glsl_count_attribute_slots(variable->type, false);
 
 	variable->data.driver_location = idx * 4;
@@ -4155,7 +4155,7 @@ handle_shader_output_decl(struct nir_to_llvm_context *ctx,
 		                       si_build_alloca_undef(ctx, ctx->f32, "");
 		}
 	}
-	ctx->output_mask |= ((1ull << attrib_count) - 1) << variable->data.location;
+	ctx->output_mask |= ((1ull << attrib_count) - 1) << idx;
 }
 
 static void