drm/panel: ili9341: fix optional regulator handling

If the optional regulator lookup fails, reset the pointer to NULL.
Other functions such as mipi_dbi_poweron_reset_conditional() only do
a NULL pointer check and will otherwise dereference the error pointer.

Fixes: 5a042273 ("drm/panel: Add ilitek ili9341 panel driver")
Signed-off-by: Daniel Mack <daniel@zonque.org>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317225537.826302-1-daniel@zonque.org