drm/i915/scaler: Clean up intel_atomic_setup_scalers() a bit

intel_atomic_setup_scalers() is currently messing around with
the internals of the atomic states. Stop doing that and instead
use the regular interfaces so that we don't need to know any ugly
implementation details.

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/20241107122658.21901-5-ville.syrjala@linux.intel.com