drm/i915: get runtime PM at i915_display_info

Otherwise we may get some WARNs complaining that we're reading a
register while we're suspended.

Testcase: igt/pm_pc8/debugfs-read
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>