i915 SSC Patch

I've added a global SSC (Spread Spectrum Clock) parameter to the i915 driver, since having SSC enabled breaks (distorts) VGA output on some Core i5/i7 chips (see https://bugs.freedesktop.org/show_bug.cgi?id=38750). SSC is still enabled by default so the behaviour won't change but setting the global_use_ssc parameter will turn this feature off and allow affected devices to function correctly (notably the Dell Vostro 3300).

Numerous people have tested this and reported it working (as seen in the bug report thread) which isn't surprising considering it's a basic modification of Chris Wilsons work.

Any comments, or anybody willing to include this patch?

Thanks, Ben Brewer (CodeThink)