drm: renesas: rz-du: mipi_dsi: fix kernel panic when rebooting for some panels

In the Linux kernel, the following vulnerability has been resolved:

drm: renesas: rz-du: mipi_dsi: fix kernel panic when rebooting for some panels

Since commit 56de5e305d4b ("clk: renesas: r9a07g044: Add MSTOP for RZ/G2L")
we may get the following kernel panic, for some panels, when rebooting:

systemd-shutdown[1]: Rebooting.
Call trace:
...
do_serror+0x28/0x68
el1h_64_error_handler+0x34/0x50
el1h_64_error+0x6c/0x70
rzg2l_mipi_dsi_host_transfer+0x114/0x458 (P)
mipi_dsi_device_transfer+0x44/0x58
mipi_dsi_dcs_set_display_off_multi+0x9c/0xc4
ili9881c_unprepare+0x38/0x88
drm_panel_unprepare+0xbc/0x108

This happens for panels that need to send MIPI-DSI commands in their
unprepare() callback. Since the MIPI-DSI interface is stopped at that
point, rzg2l_mipi_dsi_host_transfer() triggers the kernel panic.

Fix by moving rzg2l_mipi_dsi_stop() to new callback function
rzg2l_mipi_dsi_atomic_post_disable().

With this change we now have the correct power-down/stop sequence:

systemd-shutdown[1]: Rebooting.
rzg2l-mipi-dsi 10850000.dsi: rzg2l_mipi_dsi_atomic_disable(): entry
ili9881c-dsi 10850000.dsi.0: ili9881c_unprepare(): entry
rzg2l-mipi-dsi 10850000.dsi: rzg2l_mipi_dsi_atomic_post_disable(): entry
reboot: Restarting system