drm/i915: fix applying placement flag

Switching from a separate list to flags introduced a bug here.

We were accidentially ORing the flag before initailizing the placement
and not after. So this code didn't do nothing except producing a
warning.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: a78a8da5 ("drm/ttm: replace busy placement with flags v6")
Link: https://patchwork.freedesktop.org/patch/msgid/20240226142759.93130-1-christian.koenig@amd.com


Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>	# compile only
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>