drm/meson: dw-hdmi: Register a callback to disable the regulator

Removing the meson-dw-hdmi module results in the following splat:

i[   43.340509] WARNING: CPU: 0 PID: 572 at drivers/regulator/core.c:2125 _regulator_put.part.0+0x16c/0x174
[...]
[   43.454870] CPU: 0 PID: 572 Comm: modprobe Tainted: G        W   E     5.10.0-rc4-00049-gd274813a4de3-dirty #2147
[   43.465042] Hardware name:  , BIOS 2021.01-rc2-00012-gde865f7ee1 11/16/2020
[   43.471945] pstate: 80400009 (Nzcv daif +PAN -UAO -TCO BTYPE=--)
[   43.477896] pc : _regulator_put.part.0+0x16c/0x174
[   43.482638] lr : regulator_put+0x44/0x60
[...]
[   43.568715] Call trace:
[   43.571132]  _regulator_put.part.0+0x16c/0x174
[   43.575529]  regulator_put+0x44/0x60
[   43.579067]  devm_regulator_release+0x20/0x2c
[   43.583380]  release_nodes+0x1c8/0x2b4
[   43.587087]  devres_release_all+0x44/0x6c
[   43.591056]  __device_release_driver+0x1a0/0x23c
[   43.595626]  driver_detach+0xcc/0x160
[   43.599249]  bus_remove_driver+0x68/0xe0
[   43.603130]  driver_unregister+0x3c/0x6c
[   43.607011]  platform_driver_unregister+0x20/0x2c
[   43.611678]  meson_dw_hdmi_platform_driver_exit+0x18/0x4a8 [meson_dw_hdmi]
[   43.618485]  __arm64_sys_delete_module+0x1bc/0x294

as the HDMI regulator is still enabled on release.

In order to address this, register a callback that will deal with
the disabling when the driver is unbound, solving the problem.

Fixes: 161a803f ("drm/meson: dw_hdmi: Add support for an optional external 5V regulator")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116200744.495826-4-maz@kernel.org