drm/omap: hdmi: Allocate EDID in the .read_edid() operation

Bring the omapdss-specific .read_edid() operation in sync with the
drm_bridge .get_edid() operation to ease code reuse.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-29-laurent.pinchart@ideasonboard.com