drm/msm: Fix null pointer dereferences without iommu

Check if 'aspace' is set before using it as it will stay null without
IOMMU, such as on msm8974.

Fixes: bc211258 ("drm/msm/gpu: Track global faults per address-space")
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lore.kernel.org/r/20220421203455.313523-1-luca@z3ntu.xyz


Signed-off-by: Rob Clark <robdclark@chromium.org>