msm: kgsl: Do not free sharedmem if it cannot be unmapped

If sharedmem cannot be unmapped from the mmu, it can still be accessed
by the GPU. Therefore it is not safe to free the backing memory. In the
case that unmap fails, do not free it or return it to the system.

Change-Id: Iad3e86d043f129a4d71cf862865d9033d4a315e3
Signed-off-by: Lynus Vaz <quic_lvaz@quicinc.com>
(cherry picked from commit 8294c5b0)