Don't unreference FBs of pixmaps from different screens in LeaveVT

FindClientResourcesByType finds pixmaps from all screens, but trying to process ones from other screens here makes no sense and likely results in a crash or memory corruption.

Fixes: c16ff42f ("Make all active CRTCs scan out an all-black framebuffer in LeaveVT") (Ported from radeon commit 2faaecc69b127248718e759c6c98c84d56dd1b6b)

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