drm/i915: Eliminate bigjoiner boolean

Since we now have the bigjoiner_pipes bitmask the boolean
is redundant. Get rid of it.

Also, populating bigjoiner_pipes already during
encoder->compute_config() allows us to use it much earlier
during the state calculation as well. The initial aim is
to use it in intel_crtc_compute_config().

v2: Move the hweight(bigjoiner_pipes) stuff to a later patch

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com> #v1
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-12-ville.syrjala@linux.intel.com