drm/i915: Move the unclaimed mmio detection into the powerwell for KMS

Replace the large comment about requiring the powerwell for
intel_uncore_arm_unclaimed_mmio_detection() by moving the arming of the
mmio error detection into the powerwell held for modesetting. Thereby
also accomplishing the goal of only arming the mmio detection after a
full modeset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170504115508.13571-1-chris@chris-wilson.co.uk


Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>