drm/ttm: use phys_addr_t for ttm_bus_placement

Fixes ttm on platforms like PPC460 where the CPU
is in 32-bit mode, but the physical addresses are
>32 bits.

Extracted from a patch by Hans Verkuil.

Tested-by: Julian Margetson <runaway@candw.ms>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Julian Margetson <runaway@candw.ms>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>