SDL: Prevent SDL_FreeSurface from freeing the LCD buffer
In direct mode, the statically allocated Rockbox LCD framebuffer is used and should never be freed. Otherwise, ROCKBOX_VideoQuit takes care of freeing the dynamically allocated hidden buffer, used for scaling.
This fixed hanging of Rockbox when quitting an SDL app.
Change-Id: Ib9cd8c04cc0d2d6ccedaa582a5a85ef211d0273c