cmake: fix SDL_RENDER_VULKAN reporting

Also fix an error when configuring with `-DSDL_VULKAN=ON
-DSDL_RENDER_VULKAN=FALSE`: the vulkan renderer is now correctly
disabled.

cmake/sdlchecks.cmake

@@ -725,7 +725,10 @@ macro(CheckVulkan)
   if(SDL_VULKAN)
     set(SDL_VIDEO_VULKAN 1)
     set(HAVE_VULKAN TRUE)
-    set(SDL_VIDEO_RENDER_VULKAN 1)
+    if(SDL_RENDER_VULKAN)
+      set(SDL_VIDEO_RENDER_VULKAN 1)
+      set(HAVE_RENDER_VULKAN TRUE)
+    endif()
   endif()
 endmacro()