drm/radeon: fix typo in radeon_connector_is_dp12_capable()

commit af5d36539dfe043f1cf0f8b7334d6bb12cd14e75 upstream.

We were checking the ext clock rather than the display clock.

Noticed by ArtForz on IRC.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>