drm/i915: debug print all of the DPCD we have

At some point the DPCD size was increased, but the debug print not. While
at it, switch to using hex dump.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>