r600g/sb: fix value::is_fixed()

This prevents unnecessary (and wrong) register allocation in the
scheduler for preloaded values in fixed registers.

Fixes interpolation-mixed.shader_test on rv770
(and probably on all other pre-evergreen chips).

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Tested-by: Andreas Boll <andreas.boll.dev@gmail.com>

src/gallium/drivers/r600/sb/sb_valtable.cpp

@@ -255,8 +255,8 @@ void value::set_prealloc() {
 bool value::is_fixed() {
 	if (array && array->gpr)
 		return true;
-	if (chunk)
-		return chunk->is_fixed();
+	if (chunk && chunk->is_fixed())
+		return true;
 	return flags & VLF_FIXED;
 }