drm/radeon/atom: fix dithering on certain panels

We need to specify the encoder mode as LVDS for eDP
when using the Crtc_Source atom table in order to properly
set up the FMT hardware.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=73911



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org