msm: camera: Delete list entry after work has finished

While dispatching kernel callbacks, wait until the callback
work has finished before deleting the callback information
from the sync object callback linked list. Do this because
otherwise if the callback dispatch finishes before deleting
the list entry, the memory might be already freed and it will
lead to a use-after-free scenario.

CRs-Fixed: 1108613
Change-Id: Ieaab8dd95a81e018e0511fa162e4d32d37c7e99a
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>