drm/i915: Return both master and slave pipes from enabled_bigjoiner_pipes()

Return both the master and slave pipe bitmasks from
enabled_bigjoiner_pipes(). We'll have use for both during
readout soon.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220203183823.22890-10-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>