msm: vidc: Fix double list_del in rbr event processing

Do not use mbuf after kref_put_mbuf() instead get the
mbuf from the registered bufs list to use it again.
Using the same mbuf after kref_put_mbuf() might lead to
video failures.

Change-Id: I69f03ca1f9b7d607d407952e7a83d4977962cc5d
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>