Fix bug in -var-evaluate-expression

Fix bug in -var-evaluate-expression

This bug points out that if one uses -var-set-visualizer with "None"
-- to disable a pretty-printer for a varobj -- then
-var-evaluate-expression will still use pretty-printing.

This is a combination of bugs.  First, setting the visualizer does not
update the display text; and second, computing the display text should
use "raw" when Python is available but no visualizer is desired.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=11738
Reviewed-by: Keith Seitz <keiths@redhat.com>