drm/amd/pm: Allocate dummy table only if needed

Only Navi1x requires dummy read workaround. Allocate the table in VRAM
only for Navi1x.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>