media: vsp1: Fix smatch warning

drivers/media/platform/vsp1/vsp1_drm.c: drivers/media/platform/vsp1/vsp1_drm.c:336 vsp1_du_pipeline_setup_brx() error: we previously assumed 'pipe->brx' could be null (see line 244)

smatch missed that if pipe->brx was NULL, then later on it will be
set with a non-NULL value. But it is easier to just use the brx pointer
so smatch doesn't get confused.

Tested-on: Salvator-XS-ES2.0, Salvator-XS-M3N

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>