drm: vc4: Remove incorrect limit from hvs_dlist debugfs function

The debugfs function to dump dlists aborted at 256 bytes,
when actually the dlist memory is generally significantly
larger but varies based on SoC.

We already have the correct limit in __vc4_hvs_alloc, so
store it for use in the debugfs dlist function.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>