drm/i915/debugfs: remove i915_dpcd file

We've long had the more generic /dev/drm_dp_auxN devices for the same
purpose. Drop the redundant and limited DPCD debugfs file.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200117150551.9836-1-jani.nikula@intel.com