msm: sde: add buf_finish callback to clear last fd

Fd is tunneled using userptr memory type to v4l2 rotator
driver. Fd can assume the same value between multiple qbuf but
with the underlying mapping modified. However, v4l2 assumes that
if userptr of the same value are passed in, the underlying buffer
is the same and will bypass memory mapping callback. This will
cause problem for fd tunneling because the obsolete mapping is
used.

To ensure buffer mapping, add buf_finish callback to clear last
fd value before dequeuing buffer back to user client. This will
force the next queue buffer command to invoke memory mapping callback
since the incoming fd value is different from the reset value.

CRs-Fixed: 1084634
Change-Id: I932a58fc633918b151959fcbe320668a87dbc49c
Signed-off-by: Alan Kwong <akwong@codeaurora.org>