drm/komeda: prevent memory leak in komeda_wb_connector_add

In komeda_wb_connector_add if drm_writeback_connector_init fails the
allocated memory for kwb_conn should be released.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925043031.32308-1-navid.emamdoost@gmail.com