drm/msm: switch msm_kms_init_aspace() to use device_iommu_mapped()

Change msm_kms_init_aspace() to use generic function
device_iommu_mapped() instead of the fwnode-specific interface
dev_iommu_fwspec_get(). While we are at it, stop referencing
platform_bus_type directly and use the bus of the IOMMU device.

Suggested-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/489703/
Link: https://lore.kernel.org/r/20220616081106.350262-6-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>