Fixed Bug 4883, redux - connect SDL_GetDisplayDPI to UIKit_GetDisplayDPI

SDL_GetDisplayDPI was failing on iOS, as UIKit_GetDisplayDPI was
not getting assigned to SDL's internal field in SDL_VideoDevice:
GetDisplayDPI.

src/video/uikit/SDL_uikitvideo.m

@@ -102,6 +102,7 @@ UIKit_CreateDevice(int devindex)
         device->DestroyWindow = UIKit_DestroyWindow;
         device->GetWindowWMInfo = UIKit_GetWindowWMInfo;
         device->GetDisplayUsableBounds = UIKit_GetDisplayUsableBounds;
+        device->GetDisplayDPI = UIKit_GetDisplayDPI;
 
 #if SDL_IPHONE_KEYBOARD
         device->HasScreenKeyboardSupport = UIKit_HasScreenKeyboardSupport;