Once we know we need to take new forcewakes, that being
a slow operation, it does not make sense to inline that
code into every mmio accessor.

Move it to a separate function and save some code.

v2: Be explicit with noinline and remove stale comment.
    (Chris Wilson)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>