drm/i915: Tidy the tail of i915_tiling_ok()

drm/i915: Tidy the tail of i915_tiling_ok()

The current tail breaks the pattern of if (check) return false, which
can catch the reader out. If we move the gen2/3 power-of-two test into
the earlier gen2/3 branch, we can eliminate the contrary tail.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170203115036.24743-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>