msm: kgsl: Mark the end of the scatterlist

Make sure that the scatterlist is correctly terminated in order to
avoid going past the end of the list when freeing pages.

Change-Id: I5284fd8f66f2ecfb20a645b3fbcbd0553dd1fd1b
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>