drm/panfrost: Set DMA masks earlier

The DMA masks need to be set correctly before any DMA API activity kicks
off, and the current point in panfrost_probe() is way too late in that
regard. since panfrost_mmu_init() has already set up a live address
space and DMA-mapped MMU pagetables. We can't set masks until we've
queried the appropriate value from MMU_FEATURES, but as soon as
reasonably possible after that should suffice.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/64361b929a5c61d2ab9580262ecb3d369164cfcb.1556195258.git.robin.murphy@arm.com