drm/i915: Convert more trace events to DEFINE_EVENT

Convert i915_gem_object_clflush to DEFINE_EVENT, and save ~0.5K:

   text    data     bss     dec     hex filename
  13204    2732      12   15948    3e4c i915_trace_points.o.orig
  12668    2732      12   15412    3c34 i915_trace_points.o

No change in functionality.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Eric Anholt <eric@anholt.net>