msm: kgsl: Add snapshot memory region in minidump only once

As snapshot can be recollected and snapshot size can vary on
each recollection if we add snapshot region in minidump during
each recollection then we need to remove it and add it again
to ensure proper snapshot size is captured in minidump. To
avoid this add snapshot region to minidump once and with complete
snapshot memory size. Also, take care of removing it during close
to ensure minidump doesn't try to access already freed memory region.

Change-Id: I5eded015c0544a03eec835da57b6aaf64defc139
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>