drm/amd/display: Optimize link power-down when link powered externally

[Why]
When an eDP panel is powered externally by a different GPU, we don't need
to wait for hardware sequencing delays when powering down a link, as the
display is not dependent on the GPU being powered down.

[How]
This commit adds a variable 'link_powered_externally' to indicate when a
link is being powered by another GPU.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Felipe Clark <felipe.clark@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Tony Tascioglu <tony.tascioglu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>