WIP: perf/timechart: support for GPU

Show GPU as another "CPU" on the timeline, so we can see where it is
busy and idle.  Also show pending flips (green bar on the GPU), as well
as places where processes are blocked on a fence (yellow bar on the
process).

Currently a bit hacky.  And looking for drm/msm specific traces.  We
should probably make a set of generic traces that can be used by all drm
drivers, and use these trace events in perf.  (For other GPU's we
probably want a way to be able to visualize activity on multiple rings.)