drm/msm: fix memory leakage issue

Two places have memory leakage issue:
1. drm_prime_pages_to_sg create a sgt table, drm driver
   should free it when relase GEM buffer;
2. when import dma buf from ion driver which create a
   sgt table, drm driver should call function
   drm_prime_gem_destroy to free it when release GEM
   buffer

Change-Id: I770a753cdc874b68bd5e1dba058ff0a951c09701
Signed-off-by: Yajun Li <yajunl@codeaurora.org>