drm/i915/guc: Update uncore access path in flush_ggtt_writes

The preferred way to access the uncore is through the GT structure.
Update the GuC function, flush_ggtt_writes, to use this path.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <john.c.harrison@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207010033.24667-1-John.C.Harrison@Intel.com