drm/amd/display: set link->dongle_max_pix_clk to 0 on a disconnect

[Why]
Found issue in EDID Emulation where if we connect a display using
 a passive HDMI-DP dongle, disconnect it and then try to emulate
 a display using DP, we could not see 4K modes.  This was because
 on a disconnect, dongle_max_pix_clk was still set so when we
 emulate using DP, in dc_link_validate_mode_timing(), it would
 think we were still using a dongle and limit the modes we support.

[How]
In dc_link_detect(), set dongle_max_pix_clk to 0 when we detect
 a hotplug out ( if new_connection_type = dc_connection_none ).

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>