drm/xe/display: Re-use display vmas when possible

i915 has this really nice, infrastructure where everything becomes
complicated, GGTT needs eviction, etc..

Lets not do that, and make the dumbest possible interface instead.
Try to retrieve the VMA from old_plane_state, or intel_fbdev if kernel
fb.

Link: https://patchwork.freedesktop.org/patch/msgid/20241206182032.196307-1-dev@lankhorst.se


Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Tested-by: Jani Saarinen <jani.saarinen@intel.com>