drm/i915: Eliminate unused mmio accessors

On the path of removing mmio accessors with implicit dev_priv, easy first
step is to remove all such unused macros.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610120608.15477-1-tvrtko.ursulin@linux.intel.com