msm: kgsl: Add a flag to mark user mapped GPU buffers

Add a flag in /sys/kernel/debug/kgsl/kgsl-3do/proc/<pid>/mem
file to indicate for each GPU buffer if it is mapped to userspace
in <pid> or not.

CRs-fixed: 634962
Change-Id: I8abda74ef5656aca6b1c0315af8deb77460fa5a9
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>