drm/client: Stop using the legacy crtc->mode

drm/client: Stop using the legacy crtc->mode

crtc->mode is legacy junk and shouldn't really be used with
atomic drivers.

Most (all?) atomic drivers do end up still calling
drm_atomic_helper_update_legacy_modeset_state() at some
point, so crtc->mode does still get populated, and this
does work for now. But now that the modes[] lifetime issues
have been sorted out we can just switch over to the
proper crtc->state->mode.

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250228211454.8138-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>