drm/i915/dsi: Grab the crtc from the customary place

The encoder hooks already get passed the crtc state so just
grab the crtc from there instead of from the connector state.
This is generally what everyone else does, so no reason for
icl_dsi.c to be different.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230608203057.23759-13-ville.syrjala@linux.intel.com