drm/i915: Convert some trace events to DEFINE_TRACE

Use DECLARE_EVENT_CLASS to remove duplicate code:

   text    data     bss     dec     hex filename
  14655    2732      15   17402    43fa i915_trace_points.o.orig
  11625    2732      10   14367    381f i915_trace_points.o

8 events are converted:

  i915_gem_object:  i915_gem_object_{unbind, destroy}
  i915_gem_request: i915_gem_request_{complete, retire, wait_begin, wait_end}
  i915_ring:        i915_ring_{wait_begin, wait_end}

No functional change.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Eric Anholt <eric@anholt.net>