drm/i915/gt: Simplify rc6 w/a application

Quite simply we only need to check for prior corruption on enabling rc6
on module load and resume, so by hooking into the common entry points.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202110836.2342685-2-chris@chris-wilson.co.uk