drm/i915: Introduce intel_master_crtc()

Add a helper to determine the master crtc for bigjoiner usage.
Also name the variables consistently.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211022103304.24164-5-ville.syrjala@linux.intel.com