drm/i915: rename VLV IOSF sideband functions logically

Rename all VLV IOSF sideband register accessor functions to
vlv_<port>_{read,write}. No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>