kmsdrm: Fix wrong check on KMSDRM_CreateWindow.

A previous cleanup commit inverted a statement that checked the return value of
a KMSDRM_CreateSurfaces call during KMSDRM_CreateWindow, which causes the video
backend to always fail despite success.

This commit restores the intended behavior.

Fixes: 3c501b9 ("Clang-Tidy fixes (libsdl-org#6725)").

src/video/kmsdrm/SDL_kmsdrmvideo.c

@@ -1490,7 +1490,7 @@ int KMSDRM_CreateWindow(_THIS, SDL_Window *window)
         /* Create the window surfaces with the size we have just chosen.
            Needs the window diverdata in place. */
         ret = KMSDRM_CreateSurfaces(_this, window);
-        if (ret == 0) {
+        if (ret != 0) {
             return SDL_SetError("Can't window GBM/EGL surfaces on window creation.");
         }
     } /* NON-Vulkan block ends. */