perf scripts python: call-graph-from-sql.py: Set a minimum window size

perf scripts python: call-graph-from-sql.py: Set a minimum window size

Prevent weirdly small window size.

Committer testing:

Seems to work, but even before this patch, on my system, it always
started with:

xwininfo: Window id: 0x1e00002 "Call Graph: pt_example"
<SNIP>
  Width: 800
  Height: 600
<SNIP>

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181001062853.28285-4-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>