msm: kgsl: Properly record the size of the sglist in the memdesc

It was possible to fail out of kgsl_sharedmem_alloc after a sglist was
allocated but before we saved the size of the allocation which caused
problems when the sglist was freed during cleanup.

Change-Id: Ic0dedbadd57ec4d9422b5e3a873243d0ffed51f7
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>