nvfx: Fix include recursion.

Don't include nvfx_context.h and use a forward reference instead.
nvfx_context.h includes nvfx_screen.h (itself).

src/gallium/drivers/nvfx/nvfx_screen.h

@@ -4,7 +4,8 @@
 #include "util/u_double_list.h"
 #include "nouveau/nouveau_screen.h"
 #include "nv04_surface_2d.h"
-#include "nvfx_context.h"
+
+struct nvfx_context;
 
 struct nvfx_screen {
 	struct nouveau_screen base;