drm/i915: Don't sanitize has_decoupled_mmio if platform is not broxton

The check in __intel_uncore_early_sanitize() to disable decoupled mmio
would disable it for every platform that is not broxton. While that's
not a problem now since only broxton supports that, simply setting
.has_decoupled_mmio in a new platform's device info wouldn't suffice. So
avoid future confusion and change the workaround to only change the
value of has_decoupled_mmio for broxton.

v2: git add compile fix. (Ander)

Cc: Praveen Paneri <praveen.paneri@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479993807-29353-1-git-send-email-ander.conselvan.de.oliveira@intel.com