msm: kgsl: Make snapshot priority configurable

Add a setting and a corresponding sysfs node that
controls whether new snapshots overwrite old
snapshots in case they are unrecoverable. This
prioritization of unrecoverable snapshots already
exists, but we want the option to turn it off.
By default, keep this option on.

Change-Id: Ia7dfa5f0f4bbb45fcd72fe769adb366f00b0fcbc
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>