drm/msm/adreno: fix probe without iommu

The function iommu_domain_alloc returns NULL on platforms without IOMMU
such as msm8974. This resulted in PTR_ERR(-ENODEV) being assigned to
gpu->aspace so the correct code path wasn't taken.

Fixes: ccac7ce3 ("drm/msm: Refactor address space initialization")
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>