drm/etnaviv: don't truncate physical page address

While the interface for the MMU mapping takes phys_addr_t to hold a
full 64bit address when necessary and MMUv2 is able to map physical
addresses with up to 40bit, etnaviv_iommu_map() truncates the address
to 32bits. Fix this by using the correct type.

Fixes: 931e97f3 ("drm/etnaviv: mmuv2: support 40 bit phys address")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>