drm/vc4: hdmi: Add a workqueue to set scrambling

It looks like some displays (like the LG 27UL850-W) don't enable the
scrambling when the HDMI driver enables it. However, if we set later the
scrambler enable bit, the display will work as expected.

Let's create delayed work queue to periodically look at the display
scrambling status, and if it's not set yet try to enable it again.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507150515.257424-12-maxime@cerno.tech