meta: leaking of BO with DrawPixels

ctx->Unpack.BufferObj wasn't unreferenced.

Fixes: d492e7b0 (meta: Fix invalid PBO access from DrawPixels when
trying to just alloc.)
CC: Eric Anholt <eric@anholt.net>
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 3853871e)