drm: tda998x: Fix EDID read timeout on HDMI connect

When the HDMI cable is disconnected and reconnected, EDID reading
is called too early raising a EDID read timeout.
This patch uses the system work queue to delay the notification
of the HDMI connect/disconnect event.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>