drm/i915: Remove decoupled MMIO code

This is a follow-up patch to the previous patch ([PATCH[1/2] drm/i915:
Disable decoupled MMIO) to remove the dead code for decoupled MMIO
implementation, as it won't be used any longer on GEN9LP.

Therefore, this patch reverts:

commit 85ee17eb
Author: Praveen Paneri <praveen.paneri@intel.com>
Date:   Tue Nov 15 22:49:20 2016 +0530

    drm/i915/bxt: Broxton decoupled MMIO

Signed-off-by: Kai Chen <kai.chen@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523215812.18328-3-kai.chen@intel.com