drm/amdgpu: use new HMM APIs and helpers

HMM provides new APIs and helps in kernel 5.2-rc1 to simplify driver
path. The old hmm APIs are deprecated and will be removed in future.

Below are changes in driver:

1. Change hmm_vma_fault to hmm_range_register and hmm_range_fault which
supports range with multiple vmas, remove the multiple vmas handle path
and data structure.
2. Change hmm_vma_range_done to hmm_range_unregister.
3. Use default flags to avoid pre-fill pfn arrays.
4. Use new hmm_device_ helpers.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>