drm/i915: Remove POSTING_READ16

Only a few call sites remain which have been converted to uncore mmio
accessors and so the macro can be removed.

ENGINE_POSTING_READ16 is added to replace one engine->mmio_base relative
call site.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611104548.30545-3-tvrtko.ursulin@linux.intel.com