drm/i915: Fix DP-MST crtc_mask

Each fake MST encoder is tied to a specific pipe. Fix the encoder's
crtc_mask to reflect that fact.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190817093902.2171-16-lucas.demarchi@intel.com