radeonsi: assume ClipDistance usage mask is always 0xf

No code in Mesa sets the usage mask to any other value.
The final mask is AND'ed with enable bits from the rasterizer state anyway.

If somebody implements setting usage masks in st/mesa, we can use
tgsi_shader_info to get it more easily.

This is a prerequisite for the following commit.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
diff --git a/src/gallium/drivers/radeonsi/si_shader.c b/src/gallium/drivers/radeonsi/si_shader.c
index 6ac39e8..e21cb76 100644
--- a/src/gallium/drivers/radeonsi/si_shader.c
+++ b/src/gallium/drivers/radeonsi/si_shader.c
@@ -51,7 +51,6 @@
 	unsigned name;
 	unsigned index;
 	unsigned sid;
-	unsigned usage;
 };
 
 struct si_shader_context
@@ -256,7 +255,6 @@
 		shader->output[i].name = d->Semantic.Name;
 		shader->output[i].sid = d->Semantic.Index;
 		shader->output[i].index = d->Range.First;
-		shader->output[i].usage = d->Declaration.UsageMask;
 		break;
 	}
 
@@ -1135,7 +1133,7 @@
 	LLVMValueRef args[9];
 	LLVMValueRef pos_args[4][9] = { { 0 } };
 	LLVMValueRef psize_value = NULL, edgeflag_value = NULL, layer_value = NULL;
-	unsigned semantic_name, semantic_index, semantic_usage;
+	unsigned semantic_name, semantic_index;
 	unsigned target;
 	unsigned param_count = 0;
 	unsigned pos_idx;
@@ -1148,7 +1146,6 @@
 	for (i = 0; i < noutput; i++) {
 		semantic_name = outputs[i].name;
 		semantic_index = outputs[i].sid;
-		semantic_usage = outputs[i].usage;
 
 handle_semantic:
 		/* Select the correct target */
@@ -1182,7 +1179,7 @@
 			      (1 << semantic_index)))
 				continue;
 			shader->clip_dist_write |=
-				semantic_usage << (semantic_index << 2);
+				0xf << (semantic_index * 4);
 			target = V_008DFC_SQ_EXP_POS + 2 + semantic_index;
 			break;
 		case TGSI_SEMANTIC_CLIPVERTEX:
@@ -1394,7 +1391,6 @@
 			outputs[noutput].index = index;
 			outputs[noutput].name = d->Semantic.Name;
 			outputs[noutput].sid = d->Semantic.Index;
-			outputs[noutput].usage = d->Declaration.UsageMask;
 
 			for (i = 0; i < 4; i++)
 				outputs[noutput].values[i] =
@@ -2758,7 +2754,6 @@
 		outputs[i].name = out->name;
 		outputs[i].index = out->index;
 		outputs[i].sid = out->sid;
-		outputs[i].usage = out->usage;
 
 		for (chan = 0; chan < 4; chan++) {
 			args[2] = lp_build_const_int32(gallivm,