wined3d: Do not set the pixel format if wglGetPixelFormat() returns zero and we already set the internal pixel format.

Currently this does nothing, because wglGetPixelFormat() returns the pixel
format set by wglSetPixelFormatWINE(). However, with the following changes to
WGL_WINE_pixel_format_passthrough, wglGetPixelFormat() will only return the
pixel format set by wglSetPixelFormat(). Hence we should avoid trying to set
the "internal" pixel format more than once.