msm: camera: ife: pagefault_ops access protected with mutex_lock

Pagefault_ops is protected by the mutex_lock to avoid the
list corruption by concurrent access by flush calls.

CRs-Fixed: 2904307
Change-Id: Ia6519a382ace7b89b6bd59a2dacfeca3c31e2705
Signed-off-by: Tejas Prajapati <tpraja@codeaurora.org>