drm/i915/psr: Add PSR mode/revision to debugfs

Log the PSR mode/revision (PSR1 or PSR2) in the debugfs file
i915_edp_psr_status.

Suggested-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Azhar Shaikh <azhar.shaikh@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1534958628-193724-1-git-send-email-azhar.shaikh@intel.com