Tweak OS_ReadModeVariable optimisations
Detail: s/vdu/vduswis - Don't use RMVForCurrentMode when R0 = ModeNo, only use it if R0 is -1. This ensures that results for explicitly-specified modes will always represent the base properties of the mode and won't be influenced by any unusual properties of the current mode, e.g. LineLength/ScreenSize alterations caused by driver-specific framebuffer padding. Admin: Tested on Raspberry Pi 3 Version 5.83. Tagged as 'Kernel-5_83'
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