drm/i915: Clear FORCEWAKE when taking over from BIOS

Some BIOSes may forcibly suspend RC6 during their operation which
trigger a warning as we find the hardware in a perplexing state upon
first use. So far that appears to be the worst symptom as fortuituously
we use the same values as the BIOS for programming the FORCEWAKE register.

Reported-by: Oleksij Rempel <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>