drm/i915: Take runtime pm reference on hangcheck_info

drm/i915: Take runtime pm reference on hangcheck_info

We read the coherent current seqno and actual head from ring.
For hardware access we need to take runtime_pm reference.

Get hardware specific values with runtime reference held
and print them first to emphasize hw state vs bookkeepping.

v2: Reorder output according to hw access (Chris)
    remove superfluous locking (Daniel)

Testcase: igt/pm_rpm/debugfs-read
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88910
Tested-by: Ding Heng <hengx.ding@intel.com> (v1)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>