drm/i915/gt: Pull ring submission resume under its caller forcewake

Take advantage of calling xcs_resume under a forcewake by using direct
mmio access. In particular, we can avoid the sleeping variants to allow
resume to be called from softirq context, required for engine resets.

v2: Keep the posting read at the start of resume as a guardian memory
barrier.

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/20210119110802.22228-5-chris@chris-wilson.co.uk