drm/i915: Move compressed_fb to static allocation

We are already using the size to determine whether or not to free the
object, so there is no functional change there. Almost everything else
has changed to static allocations of the drm_mm_node too.

Aside from bringing this inline with much of our other code, this makes
error paths slightly simpler, which benefits the look of an upcoming
patch.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>