drm/amd: Check that a system is a NUMA system before looking for SRAT

It's pointless on laptops to look for the SRAT table as these are not
NUMA.  Check the number of possible nodes is > 1 to decide whether to
look for SRAT.

Suggested-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>