PSP fullscreen is the only mode (#13125)

(cherry picked from commit 9b025e3cab64a56331e9c02783ea833ae1211d48)

src/video/psp/SDL_pspvideo.c

@@ -118,6 +118,8 @@ static SDL_VideoDevice *PSP_Create(void)
 
     device->PumpEvents = PSP_PumpEvents;
 
+    device->device_caps = VIDEO_DEVICE_CAPS_FULLSCREEN_ONLY;
+
     return device;
 }