drm/amd/display: Fix MST daisy chain SST not light up

In SST daisy chain scenario, edid is getting read in mst hotplug
routine. It is getting conflict with drm send_enum_path_resources
kernel thread in terms of i2c bus which is getting locked up in
such case.

Have edid being read in get_mode hook, instead of in hotplug
routine.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>