drm/arm/malidp: Stop using iommu_present()

iommu_get_domain_for_dev() is already perfectly happy to return NULL
if the given device has no IOMMU. Drop the unnecessary check.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5049994e6c2ba92c2f30d51850c8929136d0f8ca.1649167878.git.robin.murphy@arm.com