drm/i915: Disable decoupled MMIO

drm/i915: Disable decoupled MMIO

The decoupled MMIO feature doesn't work as intended by HW team. Enabling
it with forcewake will only make debugging efforts more difficult, so
let's disable it.

Fixes: 85ee17ebeedd ("drm/i915/bxt: Broxton decoupled MMIO")
Cc: Zhe Wang <[email protected]>
Cc: Praveen Paneri <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v4.10+
Signed-off-by: Kai Chen <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]