plotter: EventPlot: avoid changes on input data by operating on a copy
plotter: EventPlot: avoid changes on input data by operating on a copy

_get_lanes() does some changes to the input data that are necessary for the
Javascript code to work properly. However, these changes permanently affect the
input data. In particular, "lane" names are converted to integers.

To avoid this, let's create an internal copy of the input data and always
operate on that.

Signed-off-by: Michele Di Giorgio <99fd0b6f5a4e02a4022988691284da2a0299b046@arm.com>

ARM-software/trappy,joelagnel/trappy,joelagnel/trappy,ARM-software/trappy,ARM-software/trappy,sinkap/trappy,ARM-software/trappy,bjackman/trappy,bjackman/trappy,bjackman/trappy,sinkap/trappy,sinkap/trappy,joelagnel/trappy,bjackman/trappy,sinkap/trappy,joelagnel/trappy