qseecom: Proper handling of unmapping dmabuf

After unmapping the dmabuf, the pointers for dmabuf
and sg lists should be explicitly made NULL. This
change takes care of releasing the already unmapped
dma buf pointers and sg pointers.

Change-Id: Ia33231af761eddbdd6f71fda2c16b37289724b2e
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>