drm/i915: Add some debug breadcrumbs to connector detection

Try to decypher detection failures is a little tricker at the moment as
the only indicator of progress is when output_poll_execute() tells us
the result after the connector->detect() has run. This patch adds a
telltale to the start of each detect function so that we can track
progress and associate activity more clearly with each connector.

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