drm/i915: Add i915_lpsp_status debugfs attribute

It requires a separate debugfs attribute to expose lpsp
status to user space, as there may be display less configuration
without any valid connected output, those configuration will not be
able to test lpsp status, if lpsp status exposed from a connector
based debugfs attribute.

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415170555.15531-5-anshuman.gupta@intel.com