msm: kgsl: restore usage flags on imported buffers

Commit 20ef1f47 (msm: kgsl: set usermem
type in the memdesc flags) inadvertently prevented the KGSL_MEMTYPE_MASK
bits from being set by userspace on mapped buffers. Put them back.

Change-Id: If3b3f52045136225dd31668271c29840bbbc8f99
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>