drm/amdgpu: remove dead code after hmm_range_fault()

Since amdgpu does not use the snapshot mode of hmm_range_fault() a
successful return already proves that all entries in the pfns are
HMM_PFN_VALID, there is no need to check the return result of
hmm_device_entry_to_page().

Link: https://lore.kernel.org/r/3-v2-b4e84f444c7d+24f57-hmm_no_flags_jgg@mellanox.com


Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>