drm/i915: Educate users in dmesg about reporting gpu hangs

Untangling me-too reports that actually aren't is really messy. And we
need to make sure the blame is put where it should be right from the
start ;-)

v2: Improve the wording from Ben's suggestions.

Cc: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Frob the message as suggested by Paulo on irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>