GSdx-d3d: Drop an useless min in FS - port from GL.

Commit: a1957a685d

GPU will clamp color anyway. It reduces the number of instruction of 25%
(4->3).

plugins/GSdx/res/merge.fx

@@ -18,7 +18,7 @@ struct PS_INPUT
 float4 ps_main0(PS_INPUT input) : SV_Target0
 {
 	float4 c = Texture.Sample(Sampler, input.t);
-	c.a = min(c.a * 2, 1);
+	c.a *= 2.0f;
 	return c;
 }