drm/panel: innolux-p079zca: Delete panel on attach() failure

If we fail to attach (e.g., because 1 of 2 dual-DSI controllers aren't
ready), we leave a dangling drm_panel reference to freed memory. Clean
that up on failure.

This problem exists since the driver's introduction, but is especially
relevant after refactored for dual-DSI variants.

Fixes: 14c8f2e9 ("drm/panel: add Innolux P079ZCA panel driver")
Fixes: 7ad4e463 ("drm/panel: p079zca: Refactor panel driver to support multiple panels")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210923173336.2.I9023cf8811a3abf4964ed84eb681721d8bb489d6@changeid