drm/mm: Fix NULL pointer dereference in allocator

While searching for first hole (get_first_hole) and next free hole
(get_next_hole) from rbtree, rb_first() and rb_next() functions may
return NULL pointer. A NULL pointer check can avoid the NULL pointer
dereference.

Change-Id: I21c918fff3ae995a9d8b2201030401d80ed1fd9e
Signed-off-by: Venkateswara Rao Tadikonda <vtadik@codeaurora.org>