drm/i915: improve i915_wait_request_begin trace

The trace events adds whether or not the wait was blocking. Blocking in
this case means to hold struct_mutex (ie. no new work can be submitted
during the wait). The information is inherently racy.

The blocking information is racy since mutex_is_locked doesn't check
that the current thread holds the lock. The only other option would be
to pass the boolean information of whether or not the class was blocking
down through the stack which is less desirable.

v2: Don't do a trace event per loop. (Chris)
Only get blocking/non-blocking info (Chris)

v3: updated comment in code as well as commit msg (Daniel)
Add "(NB)" to trace information to remind us in 6 months (Ben)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>