r600g: add support for constants in memory buffers.

DX9 constants were in the constant file, and evergreen no longer support
cfile. r600/700 can also use constants in memory buffers, so add the code
(disabled for now) to enable that as precursor for evergreen.

src/gallium/drivers/r600/r600_shader.h

@@ -43,6 +43,7 @@ struct r600_shader {
 	struct r600_shader_io	output[32];
 	enum radeon_family	family;
 	boolean                 uses_kill;
+	boolean                 use_mem_constant;
 };
 
 #endif