[Why]
Unused VMIDs were not evicted correctly

[How]
1. evict_vmids() logic was fixed;
2. Added boundary check for add_ptb_to_table() and
   clear_entry_from_vmid_table() to avoid crash caused by array out of
   boundary;
3. For mod_vmid_get_for_ptb(), vimd is changed from unsigned to signed
   due to vimd is signed.
Signed-off-by: Peikang Zhang <peikang.zhang@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>