drm/i915: simplify swapin/out swizzle checking a bit

Use the helper function already employed by the pwrite/pread
functions.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>