drm/radeon: allow pcie gen2 speed on Cayman

Looks like the same pcie gen2 speed initialization for
Evergreen also works on Cayman and seems to come up fine,
so enable it if the module parameter says so

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>