drm/ttm: Simplify ttm_bo_wait_unreserved

Function ttm_bo_wait_unreserved can be slightly simplified.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>