drm/exynos: Consolidate return statements in fimd_bind()

Simplify the code and remove superfluous return statement. Just return
the result of fimd_iommu_attach_devices().

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>