Transcoder A will always use PLL A and transcoder B will use PLL B.  But
transcoder C could use either, so always mask the select bits off before
or'ing in a new value.
Reported-by: NAdam Jackson <ajax@redhat.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Tested-By: NEugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-By: NEugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: NKeith Packard <keithp@keithp.com>