msm: kgsl: Don't allow re-importing memory owned by KGSL

Don't allow IOCTL_KGSL_MAP_USER_MEM to import user memory that was
already allocated and mapped by KGSL in the first place.  Remapping
memory never makes sense and it messes up reference counting in the
pools.

Change-Id: Ic0dedbade96ac6b30dcbbb794bf57a597f1bb351
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>