drm/i915: Make __intel_set_mode() take only atomic state as argument

With the use of drm_atomic_helper_update_legacy_modeset_state the
last user of modeset_crtc is removed from this function.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>