msm: kgsl: Fix memory leak for anonymous buffers

Currently, clean up is not done properly for anonymous buffer
(KGSL_MEM_ENTRY_USER). Fix it by freeing up resources allocated
during memdesc_sg_virt.

Change-Id: I75bff2e718b494c102a8075a5f27323e1823b212
Signed-off-by: Kamal Agrawal <kamaagra@codeaurora.org>