drm/amd/display: Skip backlight control delay on external powered links

[Why]
When an eDP panel is powered externally from a different GPU, we can avoid
waiting for hardware sequencing delays when switching the backlight on/off
as the display backlight is no longer powered by the original source.

[How]
This commit extends the 'link_powered_externally' variable to allow
bypassing hardware delays for additional backlight commands and force the
backlight on/off when a link is 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>