drm/panel: Delete panel on mipi_dsi_attach() failure

Many DSI panel drivers fail to clean up their panel references on
mipi_dsi_attach() failure, so we're leaving a dangling drm_panel
reference to freed memory. Clean that up on failure.

Noticed by inspection, after seeing similar problems on other drivers.
Therefore, I'm not marking Fixes/stable.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210923173336.3.If9e74fa9b1d6eaa9e0e5b95b2b957b992740251c@changeid