drm/tilcdc: panel: Use devm_kzalloc to simplify the error path

Using the managed variant to allocate the resource makes the code simpler
and less error-prone.

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>