msm: kgsl: Use a token address for memstore

The user maps the memstore using the GPU address of the shadow memory.
Unfortunately, mmap() only uses a unsigned long which means that 32 bit
applications can handle when TTBR1 is enabled and the memstore is in
very high memory.

Instead of passing the real GPU address pass a dummy 32 bit value and use
that to identify the memstore instead.

Change-Id: Ic0dedbaddd6efda305953b20cfaa4a597020bff9
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>