drm/i915: Remove I915_READ16 and I915_WRITE16

Remove call sites in favour of uncore mmio accessors and remove the old
macros.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611104548.30545-6-tvrtko.ursulin@linux.intel.com