drm/i915: add i915 specific connector debugfs file for DPCD

Occasionally it would be interesting to read some of the DPCD registers
for debug purposes, without having to resort to logging. Add an i915
specific i915_dpcd debugfs file for DP and eDP connectors to dump parts
of the DPCD. Currently the DPCD addresses to be dumped are statically
configured, and more can be added trivially.

The implementation also makes it relatively easy to add other i915 and
connector specific debugfs files in the future, as necessary.

This is currently i915 specific just because there's no generic way to
do AUX transactions given just a drm_connector. However it's all pretty
straightforward to port to other drivers.

v2: Add more DPCD registers to dump.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>