drm/i915/psr: Remove PSR active flag from debugfs

The flag becomes misleading with flips and cursor moves not modifying it's
state as HW takes care of exiting PSR (when HW tracking is enabled)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313044211.27105-1-dhinakaran.pandiyan@intel.com