drm/amdkfd:remove unused code

Function svm_range_split_by_grinity is not used,
so it is removed.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>