drm/i915: debugfs: Add support for probing DP sink CRC.

drm/i915: debugfs: Add support for probing DP sink CRC.

This debugfs interface will allow intel-gpu-tools test case
to verify if screen has been updated properly on cases like PSR.

v2: Accepted all Daniel's suggestions:
    * grab modeset lock
    * loop over connector and check DPMS on
    * return errors
    * use _eDP1 suffix for easy future extension
    * don't cache crc_supported neither latest crc
    * return crc as a full array and read it at once with aux.
    * use 0 to turn TEST_SINK off.
    * split the drm_helpers definitions in another patch.

v3: Accepted 2 Damien's suggestion: remove h from printf hexa
    and return ENODEV when eDP not present instead of EAGAIN.

v4: Accepted 2 Jani' s suggestion: 1 path for unlock and remove
    _retry from aux read.

v5: removing last missing useless _retry (by Damien)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>