drm/amd/display: populate subvp cmd info only for the top pipe

[Why]
System restart observed while changing the display resolution
to 8k with extended mode. Sytem restart was caused by a page fault.

[How]
When the driver populates subvp info it did it for both the pipes using
vblank which caused an outof bounds array access causing the page fault.
added checks to allow the top pipe only to fix this issue.

Co-authored-by: Ayush Gupta <ayush.gupta@amd.com>
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Ayush Gupta <ayush.gupta@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>