drm/radeon: fix doublescan modes (v2)

commit fd99a094

 upstream.

Use the correct flags for atom.

v2: handle DRM_MODE_FLAG_DBLCLK

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