drm/amdgpu: fix before and after mapping judgement for replace mapping

If the before mapping is 1 page size, so its start and last will be same.
Thus below condition will become false, then to free the before mapping.
   > if (before->it.start != before->it.last)
But in this case, we need the before mapping of 1 page size.
So does after mapping.

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>