drm/i915: fix DSPADDR Gen check

The first version of commit "drm/i915: there's no DSPADDR register on
Haswell" added 2 "!IS_HASWELL" checks. When reviewing the patch, Ben
suggested to make these checks more future-proof, so when Daniel
applied the patch he fixed the first check but not the second. This
commit makes the second check also "future-proof".

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>