drm/i915: add some runtime PM get/put calls

These are needed when we cat the debugfs and sysfs files.

V2: - Rebase
V3: - Rebase
V4: - Rebase

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>