drm/amdkfd: Remove svm range validated_once flag

The validated_once flag is not used after the prefault was removed, The
prefault was needed to ensure validate all system memory pages at least
once before mapping or migrating the range to GPU.

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>