nv50: fix TEX for WriteMask not equal 0xf

If you e.g. only need alpha, it ends up in the first reg,
not the last, as it would when reading rgb too.

src/gallium/drivers/nv50/nv50_program.c

@@ -1166,10 +1166,11 @@ emit_tex(struct nv50_pc *pc, struct nv50_reg **dst, unsigned mask,
 	emit(pc, e);
 
 #if 1
-	if (mask & 1) emit_mov(pc, dst[0], t[0]);
-	if (mask & 2) emit_mov(pc, dst[1], t[1]);
-	if (mask & 4) emit_mov(pc, dst[2], t[2]);
-	if (mask & 8) emit_mov(pc, dst[3], t[3]);
+	c = 0;
+	if (mask & 1) emit_mov(pc, dst[0], t[c++]);
+	if (mask & 2) emit_mov(pc, dst[1], t[c++]);
+	if (mask & 4) emit_mov(pc, dst[2], t[c++]);
+	if (mask & 8) emit_mov(pc, dst[3], t[c]);
 
 	free_temp4(pc, t);
 #else