drm/ttm: fix broken merge between drm-next and drm-misc-next

drm-next reverted the changes to ttm_tt_create() to do the
NULL check inside the function, but drm-misc-next adds new
users of this approach.

Re-apply the NULL check change inside the function to fix this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/386628/