drm/msm: correct NULL pointer dereference in context_init

Correct attempted NULL pointer dereference in context_init() when
running without an IOMMU.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Brian Masney <masneyb@onstation.org>
Fixes: 295b22ae ("drm/msm: Pass the MMU domain index in struct msm_file_private")
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190627020515.5660-1-masneyb@onstation.org