msm: kgsl: Use pid struct to find the process to reclaim

Use pid struct instead of tgid to find the correct process private
to be used for reclaim to avoid any sort of issues related to pid reuse.

Change-Id: Ifea6583fb24c6c623d54312865b3fd584ee9fd26
Signed-off-by: Puranam V G Tejaswi <pvgtejas@codeaurora.org>