drm/amdgpu: fix amdgpu_hmm_range_get_pages

drm/amdgpu: fix amdgpu_hmm_range_get_pages

The notifier sequence must only be read once or otherwise we could work
with invalid pages.

While at it also fix the coding style, e.g. drop the pre-initialized
return value and use the common define for 2G range.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Tested-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>