drm/i915: Use '1' instead of FORCEWAKE_KERNEL for ST force wake

Use the number '1' instead of FORCEWAKE_KERNEL when requesting single
thread force wake since there is only one bit in the register. Using
the FORCEWAKE_KERNEL name might give someone the wrong impression.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>