drm/amdkfd: remove logically dead code

idr_for_each_entry can ensure that mem is not empty during the loop.
So don't need check mem again.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>