drm/i915: Compartmentalize i915_ggtt_cleanup_hw

Continuing on the theme of better logical organization of our code, make
the first step towards making the ggtt code better isolated from wider
struct drm_i915_private.

v2:
 * Cleanup of mm.wc_stash does not need struct_mutex. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-26-tvrtko.ursulin@linux.intel.com