Updating the screen pixmap across mode change

[Chris Wilson]

Earlier I sent a couple of patches that fixed an issue I encounter when
the driver adjusted details of the ScreenPixmap during resize. Keith saw
that the second patch was just furthering the misery of
ScrnInfo->pixmapPrivate and suggested that it would be easier to remove it
than to continue to workaround it. The third patch does that and could be
squashed with the second if nobody sees the value in keep the sanity check
of a NULL dereference for the framebuffer across VT switch.
-Chris