MFC r270299 (dumbbell): vt_vga: When clearing video memory, don't read from it
MFC r270299 (dumbbell): vt_vga: When clearing video memory, don't read from it

  The goal is to clear the video memory, in case an application drew to
  it. So the content shouldn't be loaded in the latches, it can't be
  trusted anyway.

  This improves a bit the window switch speed.

Sponsored by:	The FreeBSD Foundation

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