drm: Fix drm_mode_attachmode_crtc()

Change drm_mode_attachmode_crtc() to take an "all or nothing" approach.
If an error is returned, there are no side effects visible.

Also change the function to always duplicate the mode passed in.

Also change the function to not give up when it finds the first
connector without and encoder.

A simpler approach would be to just remove the function completely as
it's unused currently.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>