drm/amd/display: determine if a pipe is synced by plane state

[why]
is_blanked is not a general indicator of if a pipe is synced
for all asics.
plane state is more accurate and applicable for all asics.

[how]
Remove is_blanked call and
add checking plane_state against NULL instead.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>