drm/i915/debugfs: Print sink PSR status

IGT tests could be improved with sink status, knowing for sure that
hardware have activate or exit PSR.

v3:
Reading i915_edp_psr_status was causing PSR to exit but now with
'drm/i915/psr: Prevent PSR exit when a non-pipe related register is
written' it is fixed.

Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180425212334.21109-3-jose.souza@intel.com