drm/tilcdc: panel: Set return value explicitly

Instead of setting an initial value for the return code, set it explicitly
on each error path. This is just a cosmetic cleanup, as preparation for the
enable GPIO support.

Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Johannes Pointner <johannes.pointner@br-automation.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Dave Airlie <airlied@redhat.com>