drm/amd/display: Cache backlight_millinits in link structure and setting brightness accordingly

[Why]
Need to save the cached backlight level so that display lights up using
appropriate brightness level instead of the default brightness when
waking up from s0i3.

[How]
Adding a backlight level cache in link structure. Also instead on
calling set_default_brightness_aux, check if cached values exists.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Nicholas Susanto <nicholas.susanto@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>