D3D11: Use appropriate shader for PEEK_Z.

Trying to use GetDepthMatrixProgram outside of
TCacheEntry::FromRenderTarget is a bad idea, so don't.  Instead, use a
shader which only copies the input.

Fixes lens flare depth test in Twilight Princess. See
http://code.google.com/p/dolphin-emu/issues/detail?id=5999 .