drm/i915: remove the i915_dpio debugfs entry

There are igt tools that can read/write the DPIO registers, so having a
debugfs entry for only some of those registers is somewhat arbitrary /
redundant. Remove it.

v2:
- instead of fixing the entry by taking a power domain reference around
  the register accesses, remove the entry (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>