drm/panfrost: Open/close the perfcnt BO

Commit a5efb4c9 ("drm/panfrost: Restructure the GEM object creation")
moved the drm_mm_insert_node_generic() call to the gem->open() hook,
but forgot to update perfcnt accordingly.

Patch the perfcnt logic to call panfrost_gem_open/close() where
appropriate.

Fixes: a5efb4c9 ("drm/panfrost: Restructure the GEM object creation")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129135908.2439529-6-boris.brezillon@collabora.com