drm/i915: Move fence register tracking from i915->mm to ggtt

drm/i915: Move fence register tracking from i915->mm to ggtt

As the fence registers only apply to regions inside the GGTT is makes
more sense that we track these as part of the i915_ggtt and not the
general mm. In the next patch, we will then pull the register locking
underneath the i915_ggtt.mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073254.24048-1-chris@chris-wilson.co.uk