msm: kgsl: Access map_count only if entry is successfully allocated

In kgsl_mem_entry_create, access map_count only if entry is allocated
successfully to avoid invalid access.

Change-Id: I57bce1aec2da6a27b6d13dbee96ed86a45c9660c
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>