drm/amd/display: expand the non standard link rate for testing

[Why]
6.75 Gbps link rate training for DP_TEST_LINK_RATE_RATE_8

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Allen Pan <allen.pan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>