Update maxClock to allow highrefresh rates at higher resolutions.

Open A M Reddy's requested to merge amreddys/xf86-video-dummy:patch-1 into master

The given maxClock only allows upto 4k 24Hz which hurts the refresh rate and causes tearing. This update will make it so that higher resolutions with 60Hz or high refresh rates are possible without a max limit.

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