drm/i915: Add a timing breadcrumb to panel waits

When inspecting reports that boot/suspend/resume times are unusual it
would be useful to clearly identify the time we must spend waiting for
the hardware to complete its task. In this case we have a notification
before we start waiting for the panel to change state, but none
afterwards - which would be useful.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>