msm: kgsl: Protect the mem_entry list during deletion

Use the spinlock to prevent corruption of the mem_entry list during
deletion. Otherwise the mem_entry list may be modified by another
thread at the same time resulting in memory corruption.

Change-Id: I5728a8b925753d5fda82413615e5ebaf0366dfd8
CRs-fixed: 489224
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>