drm/i915: move VLV IOSF SB unit specific helpers under display

drm/i915: move VLV IOSF SB unit specific helpers under display

Now that all the VLV IOSF SB unit specific helper users are under
display, relocate the helpers themselves under display as
well. Resurrect the vlv_sideband.[ch] name for this. Make everything
except DPIO helpers static inlines, as their implementations are
trivial.

All of this considerably simplifies the xe compat header.

v2: Rebase

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/e86c2498c9f1c1d30f8e83fa5f1c23526b87b9ab.1747061743.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>