drm/panfrost: Update create_bo flags comment

Update a comment stating create_bo took no flags, since it now takes a
bit mask of optional flags NOEXEC and HEAP.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220109163704.2564-1-alyssa.rosenzweig@collabora.com