Prevent D3D12 debug layer error with env.maps

Prevent the debug layer message

  Placed resources, reserved resources, or committed resources with
  D3D12_HEAP_FLAG_CREATE_NOT_ZEROED flag with either render target
  or depth stencil flags must be initialized with a
  Discard/Clear/Copy operations before other operations are supported.

To this by doing something (anything, like a clear) with the unused
mip levels.

Pick-to: 6.12 6.11 6.8
Change-Id: If6fb1e6854e7a10d81a04fa6dcadf38cce4a1c75
Reviewed-by: Andy Nichols <andy.nichols@qt.io>