i915: Add and use pr_fmt and pr_<level>

Use a more current logging style.  Ensure that appropriate
logging messages are prefixed with "i915: ".

Convert printks to pr_<level>.  Align arguments.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>