kmsdrm: Added a hint to specify device index.

Fixes #2811.

src/video/kmsdrm/SDL_kmsdrmvideo.c

@@ -218,11 +218,17 @@ KMSDRM_CreateDevice(void)
     SDL_VideoDevice *device;
     SDL_VideoData *viddata;
     int devindex = 0;  /* !!! FIXME: let app/user specify this. */
+    const char *hint;
 
     if (!KMSDRM_Available()) {
         return NULL;
     }
 
+    hint = SDL_GetHint(SDL_HINT_KMSDRM_DEVICE_INDEX);
+    if (hint) {
+        devindex = SDL_atoi(hint);
+    }
+
     if (!devindex || (devindex > 99)) {
         devindex = get_driindex();
     }