drm/i915: Add i915 register dumping debugfs file

Add a debugfs file to dump the entire register range. Here we
assume that reading write-only/reserved registers won't make the chip
angry. Seems to hold true, thankfully.

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>