gallium: add opcodes/cap for fine derivative support

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)
Reviewed-by: Roland Scheidegger <sroland@vmware.com> (v1)
v2: Reuse opcode gaps as suggested by Marek

src/gallium/include/pipe/p_shader_tokens.h

@@ -352,7 +352,10 @@ struct tgsi_property_data {
 #define TGSI_OPCODE_UIF                 75
 #define TGSI_OPCODE_ELSE                77
 #define TGSI_OPCODE_ENDIF               78
-                                /* gap */
+
+#define TGSI_OPCODE_DDX_FINE            79
+#define TGSI_OPCODE_DDY_FINE            80
+
 #define TGSI_OPCODE_PUSHA               81
 #define TGSI_OPCODE_POPA                82
 #define TGSI_OPCODE_CEIL                83