Tidy up the routines for interacting with the GT (in particular the
forcewake dance) which are scattered throughout the code in a single
structure.

v2: use wait_for_atomic for polling.

v3: *really* use wait_for_atomic for polling.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NEugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>