Merge VideoBootStrap::available into VideoBootStrap::create

The two are only ever called together, and combining them makes it possible
to eliminate redundant symbol loading and redundant attempts to connect
to a display server.

src/video/raspberry/SDL_rpivideo.c

@@ -50,12 +50,6 @@
 #include "SDL_rpiopengles.h"
 #include "SDL_rpimouse.h"
 
-static int
-RPI_Available(void)
-{
-    return 1;
-}
-
 static void
 RPI_Destroy(SDL_VideoDevice * device)
 {
@@ -150,7 +144,6 @@ RPI_Create()
 VideoBootStrap RPI_bootstrap = {
     "RPI",
     "RPI Video Driver",
-    RPI_Available,
     RPI_Create
 };