drm/i915: Add missing trace point to LRC execbuff code path

There is a trace point in the legacy execbuffer execution path that is missing
from the execlist path. Trace points are extremely useful for debugging and are
used by various automated validation tests. Hence, this patch adds the missing
trace point back in.

OTC-Jira: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>