drm/i915/display: Support Multiple Transcoders' PSR status on debugfs

In order to support the PSR state of each transcoder, it adds
i915_psr_status to sub-directory of each transcoder.

v2: Change using of Symbolic permissions 'S_IRUGO' to using of octal
    permissions '0444'
v5: Addressed JJani Nikula's review comments
 - Remove checking of Gen12 for i915_psr_status.
 - Add check of HAS_PSR()
 - Remove meaningless check routine.

Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210204134015.419036-2-gwan-gyeong.mun@intel.com