SMEM instructions can reach addresses above 47 bits but require
bit 47 to be sign-extended through bits [63:48].

This allows the TMA to be relocated in a following patch.
Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>