msm: kgsl: Fix pagetable member of struct kgsl_memdesc

memdesc->pagetable is supposed to help ensure that memory gets
unmapped before it is freed, but the pagetable member is being
populated at create time not when the buffer gets mapped. This
forces the developer to ensure that the same pagetable is
used for both the create and map step. Instead, assign the
pagetable member when it is first used (to get a GPU address)
and put it away when the GPU address is released.

Change-Id: Ic0dedbad372fd9029b932dd99633a650049751ed
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>