media: vsp1: wpf: Fix partition configuration for display pipelines

When configuring partitions for memory-to-memory pipelines, the WPF
accesses data of the current partition through pipe->partition.
Writeback support will require full configuration of the WPF while not
providing a valid pipe->partition. Rework the configuration code to fall
back to the full image width in that case, as is already done for the
part of the configuration currently relevant for display pipelines.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>