drm/i915: get/put runtime PM in more places at i915_debugfs.c

These are places where we read (not write) registers while we're
runtime suspended.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>