drm/meson: dw-hdmi: Disable clocks on driver teardown

The HDMI driver request clocks early, but never disable them, leaving
the clocks on even when the driver is removed.

Fix it by slightly refactoring the clock code, and register a devm
action that will eventually disable/unprepare the enabled clocks.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120094205.525228-2-maz@kernel.org