drm/i915: rewrite VLV IOSF SB unit specific read/write functions

drm/i915: rewrite VLV IOSF SB unit specific read/write functions

Rewrite the VLV IOSF SB unit specific helpers in terms of the new
generic read/write functions. They become even simpler than they were.

The DPIO get/put helpers need to get/put both DPIO units.

v2: get/put both DPIO units

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> # v1
Link: https://lore.kernel.org/r/df97dafa0f7b665e2078c392f0dc3edc59655b0a.1747061743.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>