drm/amdgpu: Fix Vega10 VM initialization

adev->family is not initialized yet when amdgpu_get_block_size is
called. Use adev->asic_type instead.

Minimum VM size is 512GB, not 256GB, for a single page table entry
in the root page table.

gmc_v9_0_vm_init is called after adev->vm_manager.max_pfn is
initialized. Move the minimum VM-size enforcement ahead of max_pfn
initializtion. Cast to 64-bit before the left-shift.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>