drm/panel/panel-sitronix-st7701: Remove panel on DSI attach failure

In case mipi_dsi_attach() fails, call drm_panel_remove() to
avoid memory leak.

Fixes: 849b2e3f ("drm/panel: Add Sitronix ST7701 panel driver")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014231106.468063-1-marex@denx.de