drm/i915/scaler: Carve up intel_atomic_setup_scalers()

Declutter intel_atomic_setup_scalers() a bit by splitting
the crtc scaling/pfit vs. plane scaling cases into their
own functions.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241107122658.21901-7-ville.syrjala@linux.intel.com