drm/i915/ipc: move IPC debugfs to skl_watermark.c

Follow the new direction for debugfs files, moving the details where the
implementation is. It seems quite natural skl_watermark.c is the place
that controls IPC details, even for debugfs, not
intel_display_debugfs.c.

Rename the functions and convert dev_priv->i915 while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9d59b18f5dc06e86a48c1ce0f40d625f51e3e85a.1662983005.git.jani.nikula@intel.com