drm/radeon: fix logic error in atombios_encoders.c

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>
Cc: stable@vger.kernel.org
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>