ANDROID: dma-buf: heaps: Don't lock unused dmabuf_page_pool mutex

On backports where the dmabuf_page_pool mutex could not be completely
removed because of KMI constraints, we lock the mutex to prevent anyone
from acquiring it. While this is fine for regular builds, lockdep
reports a deadlock because of this behavior. So here we remove the call
to mutex_lock for this unused mutex.

Bug: 269576779
Bug: 271556415
Fixes: d55aeb40 ("BACKPORT: ANDROID: dma-buf: heaps: replace mutex lock with spinlock")
Change-Id: Ic426b6e435ec4ac7b0355c50ec9ea14d38acfa43
Signed-off-by: T.J. Mercier <tjmercier@google.com>