drm/radeon: fix logic error in atombios_encoders.c

commit b9196395c905edec512dfd6690428084228c16ec upstream.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=50431

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>