drm/i915: Take forcewake once for the entire GMBUS transaction

As we do many register reads within a very short period of time, hold
the GMBUS powerwell from start to finish.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160819164503.17845-1-chris@chris-wilson.co.uk
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>