d3d12: Shrink 2D array size so that max-layer cube arrays can be created

Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14342>

src/gallium/drivers/d3d12/d3d12_screen.cpp

@@ -164,7 +164,8 @@ d3d12_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
       return 1;
 
    case PIPE_CAP_MAX_TEXTURE_ARRAY_LAYERS:
-      return D3D12_REQ_TEXTURE2D_ARRAY_AXIS_DIMENSION;
+      /* Divide by 6 because this also applies to cubemaps */
+      return D3D12_REQ_TEXTURE2D_ARRAY_AXIS_DIMENSION / 6;
 
    case PIPE_CAP_DEPTH_CLIP_DISABLE:
       return 1;