drm/panfrost: Expose a helper to trigger a GPU reset

Expose a helper to trigger a GPU reset so we can easily trigger reset
operations outside the job timeout handler.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-8-boris.brezillon@collabora.com