Disable Framebuffer Compression on 915GM and 945GM.

While this feature at best can save 05w idle power, it is very flakey. Disabling
it fixes (or at the least masks) the problem that some 915 and 945 users were
seeing where they ended up with a blank screen (it turns out that no problems
fired because everything was still working, just FBC screwed up and never
actually scanned out the framebuffer).

As some examples for reliability linux with kms has disabled fbc on these
chipsets due to some unreliability issues (especially over suspend). and the
window driver apparently also does not used it.

Tested by kettenis@, Tom Murphy, edd@ and myself.

ok matthieu@

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