drm/amd/display: disable seamless boot for external DP

[Why]
Primary feature use case is with eDP panels.

[How]
Fail seamless boot validation if display is not an eDP panel.

Signed-off-by: Anthony Wang <anthony1.wang@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>