drm/i915/scaler: Extract intel_allocate_scaler()

intel_atomic_setup_scaler() is a mess. Make it a bit less so
by moving the scaler allocation loop into its own function.

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/20241107122658.21901-9-ville.syrjala@linux.intel.com