msm: kgsl: The memory entry pending flag should be guarded

Guard the memory entry free pending flag with the process
private lock. This usage of this flag is guarded everywhere
else in the code with the same lock.

Change-Id: Iac27f08876bfe8ff6205edc8233b9c27c6e9a599
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>