drm/msm: correct attempted NULL pointer dereference in debugfs

msm_gem_describe() would attempt to dereference a NULL pointer via the
address space pointer when no IOMMU is present. Correct this by adding
the appropriate check.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Fixes: 575f0485 ("drm/msm: Clean up and enhance the output of the 'gem' debugfs node")
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190513234105.7531-2-masneyb@onstation.org