drm/i915/fbc: Embed the compressed_llb node

Not much point in dynamically allocating the line length
buffer mm node that I can see. Just embed it directly like
we do the for the cfb node. One less failure point to worry
about.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610183237.3920-4-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>