drm/bridge: analogix_dp: Don't read EDID if panel present

If there's a panel connected to the analogix_dp bridge, rely on
the panel driver for modes, rather than reading EDID *and* calling
get_modes() on the panel.

This allows panels with a valid EDID to read it in the panel driver
(e.g. simple_panel), and panels with invalid EDID to homebrew modes
in their get_modes implementation.

Signed-off-by: Sean Paul <seanpaul@chromium.org>