ddraw: Fix filtering of enumerated display modes by bits per pixel

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=10648

Your paranoid android.


=== WNT4WSSP6 (32 bit ddrawmodes) ===
ddrawmodes.c:520: Test failed: EnumDisplayModes returned: 80070057

=== W7PROX64 (64 bit ddrawmodes) ===
Timeout