drm/exynos: return NULL if exynos_pages_to_sg fails

exynos_pages_to_sg() internally calls sg_kmalloc() which can return
no pages when the system is under high memory crunch. One such instance
is chromeos-install in the chromeos. This patch adds check for the return
value of the function in subject to return NULL on failure.

Change-Id: I1b3c66caa42f1e1a36b4362dace8657bd5f47908
Signed-off-by: Subash Patel <subash.ramaswamy@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>