drm/ttm: Delete an unnecessary check before the function call "ttm_tt_destroy"

The ttm_tt_destroy() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/54338f58-830c-a8b4-4554-5d4459bcd321@users.sourceforge.net