drm/i915: use the correct force_wake function at the PC8 code

When I submitted the first patch adding these force wake functions,
Chris Wilson observed that I was using the wrong functions, so I sent
a second version of the patch to correct this problem. The problem is
that v1 was merged instead of v2.

I was able to notice the problem when running the
debugfs-forcewake-user subtest of pm_pc8 from intel-gpu-tools.

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>