drm/i915: Add tracepoints

By adding tracepoint equivalents for WATCH_BUF/EXEC we are able to monitor
the lifetimes of objects, requests and significant events. These events can
then be probed using the tracing frameworks, such as systemtap and, in
particular, perf.

For example to record the stack trace for every GPU stall during a run, use

  $ perf record -e i915:i915_gem_request_wait_begin -c 1 -g

And

  $ perf report

to view the results.

[Updated to fix compilation issues caused.]
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Ben Gamari <bgamari@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>