gpu: ion: Free the sgtable for CMA based heaps

When an sgtable is allocated with sg_alloc_table, the table needs
to be destroyed in addition to being freed. Destroy the sgtable
when freeing the associated memory.

Change-Id: Iedba81146ba453070790f071848f86826f7eec9c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>