drm/i915: Improve PSR debugfs status.

Now we have the active/inactive state for exit and this actually changes the
HW enable bit the status was a bit confusing for users. So let's provide
more info.

Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>