vnc: Default image writer type set to Auto

vnc: Default image writer type set to Auto

This commit modifies the default behaviour of the vnc frame dumping
process: rather than using 'Bitmap' as a default parameter value, it is
using 'Auto'.  Auto parameter is letting gem5 to choose the most
efficient image format among the available ones.

Change-Id: I3c8e2b5a34a5925d24892880ac362dfe38de36e3
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5182
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>