drm/panel: simple: Skip error message on deferred probe

When enable_gpio is provided via an I2C or SPI expander, it may not be
available when panel-simple probes leading to the following error:

panel-simple panel: failed to request GPIO: -517

As this error message is not very useful to the end user, skip printing
it in the case of deferred probe.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1498857286-10820-1-git-send-email-festevam@gmail.com