drm/exynos: Properly propagate return value in drm_iommu_attach_device()

Propagate the proper error codes from the called functions instead of
unconditionally returning 0.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Merge conflict so merged it manually.
Signed-off-by: Inki Dae <inki.dae@samsung.com>