drm: bridge: adv7511: Split EDID read to a separate function

To prepare for the implementation of the DRM bridge connector
operations, move EDID read out of adv7511_get_modes() to a separate
function.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526011505.31884-2-laurent.pinchart+renesas@ideasonboard.com