util: Clean up header file inclusion in u_upload_mgr.h.

Remove p_defines.h.
Remove unnecessary forward declarations.
Add forward declaration for pipe_context.

src/gallium/auxiliary/util/u_upload_mgr.h

@@ -32,11 +32,8 @@
 #ifndef U_UPLOAD_MGR_H
 #define U_UPLOAD_MGR_H
 
-#include "pipe/p_defines.h"
-
-struct pipe_screen;
+struct pipe_context;
 struct pipe_resource;
-struct u_upload_mgr;
 
 
 struct u_upload_mgr *u_upload_create( struct pipe_context *pipe,