drm/i915/skl: Add a debugfs file to dump the DDB allocation

v2: minor conflict in i915_debugfs.c
v3: Rebase on top of the for_each_pipe() change adding dev_priv as first
    argument.
v4: minor conflict in the i915_debugfs_files array
v5: minor conflict in the i915_debugfs_files array

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>