drm/i915: Use bigjoiner_pipes more

Replace the hardcoded 2 pipe assumptions when we're massaging
pipe_mode and the pipe_src rect to be suitable for bigjoiner.
Instead we can just count the number of pipes in the bitmask.

v2: Introduce intel_bigjoiner_num_pipes()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-13-ville.syrjala@linux.intel.com


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