KCachegrind: Draw a percentage bar not larger than 100%

With some profiles, the bar could be drawn that large
that this resulted in

X Error: BadDrawable (invalid Pixmap or Window parameter) 9
Major opcode: 62
Minor opcode: 0
Resource id: 0x3001217

errors from X. We get rid of this by drawing a maximum of
100% for the bar. For larger values, we still draw the
number...