drm/i915: Add breadcrumbs for why the backlight is being set

At the moment we have 3 paths that lead to actually_set_backlight(),
from modesetting, ACPI/OpRegion requests and our very own
intel_backlight interface, and we have no way of distinguishing them in
the debug log. So add a debug breadcrumb to explain the source of the
backlight changes.

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