msm: kgsl: Fix NULL pointer dereference in gmu_memory_close()

gmu_memory_probe() will happen after gmu_clock_probe(). Incase
devm_clk_get() returns error in gmu_probe, which can result into
NULL pointer dereference in gmu_memory_close(). So add a NULL
check before calling any iommu APIs from gmu_memory_close().

Change-Id: I4a0e3b839ddaa4e70d939700aa8bffa628f5f4ea
Signed-off-by: Sravankumar bijili <csbijil@codeaurora.org>