gpu: ion: Allocate the sg_table at creation time rather than dynamically

Rather than calling map_dma on the allocations dynamically, this patch
switches to creating the sg_table at the time the buffer is created.
This is necessary because in future updates the sg_table will be used
for cache maintenance.

Change-Id: I49aac7c6d3a5afc440d18b917ae0e73be5d3f56d
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>