msm: kgsl: Release all memory entries at process close

During process close release memory entries of all the file
descriptors. Also remove device file from mem entry since its
no longer used.

CRs-Fixed: 762691
Change-Id: Ib60324218454f87905d351a7c71c52e522725070
Signed-off-by: Rajeev Kulkarni <krajeev@codeaurora.org>