drm/i915/chv: Configure crtc_mask correctly for CHV

On CHV pipe C can driver only port D, and pipes A and B can drivbe only
ports B and C. Configure the crtc_mask appropriately to reflect that.

v2: Moar braces (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>