drm/amd/display: Add disconnect case on dongle check

[why]
In the case of an external monitor disconnection,
the kernel mode will attempt to post new timing
validation with two path counts (eDP + external
monitor removed to virtual).

[how]
Skip validating color depth and pixel encoding in the
scenario involving a DP to HDMI active converter dongle.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Jingwen Zhu <Jingwen.Zhu@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>