drm/i915: fix transcoder PLL select masking

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: Adam Jackson <ajax@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-By: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>