drm/panfrost: Remove commented out call to panfrost_core_dump

panfrost_core_dump() has never existed in mainline, so remove it and add
a TODO entry that core dump support is currently lacking.

Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009094456.9704-2-steven.price@arm.com