drm/msm: Support pgtable preallocation

Introduce a mechanism to count the worst case # of pages required in a
VM_BIND op.

Note that previously we would have had to somehow account for
allocations in unmap, when splitting a block.  This behavior was removed
in commit 33729a5f ("iommu/io-pgtable-arm: Remove split on unmap
behavior)"

Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661515/