drm/i2c: tda998x: use drm_do_get_edid()

Replace the internal EDID read implementation by a call to the new EDID
read core function.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>