drm/i915/reg: abstract display_mmio_offset access

Add a macro wrapper for display_mmio_offset access in register
definitions. Prep work for reducing direct dev_priv->info usage. No
functional changes.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/aa4e8fd85e0445ec5be6c55151239072b4315fda.1546267488.git.jani.nikula@intel.com