gallium: fix the front face semantics

mesa allocates both frontface and pointcoord registers within the fog
coordinate register, by using swizzling. to make it cleaner and easier
for drivers we want each of them in its own register. so when doing
compilation from the mesa IR to tgsi allocate new registers for both
and add new semantics to the respective declarations.

src/gallium/include/pipe/p_shader_tokens.h

@@ -131,7 +131,8 @@ struct tgsi_declaration_range
 #define TGSI_SEMANTIC_PSIZE    4
 #define TGSI_SEMANTIC_GENERIC  5
 #define TGSI_SEMANTIC_NORMAL   6
-#define TGSI_SEMANTIC_COUNT    7 /**< number of semantic values */
+#define TGSI_SEMANTIC_FACE     7
+#define TGSI_SEMANTIC_COUNT    8 /**< number of semantic values */
 
 struct tgsi_declaration_semantic
 {