drm/panfrost: perfcnt: Reserve/use the AS attached to the perfcnt MMU context

We need to use the AS attached to the opened FD when dumping counters.

Reported-by: Antonio Caggiano <antonio.caggiano@collabora.com>
Fixes: 7282f764 ("drm/panfrost: Implement per FD address spaces")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Tested-by: Antonio Caggiano <antonio.caggiano@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200206141327.446127-1-boris.brezillon@collabora.com