drm/vc4: Fix leak of the file_priv that stored the perfmon.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 65101d8c

 ("drm/vc4: Expose performance counters to userspace")
Link: https://patchwork.freedesktop.org/patch/msgid/20180409205813.7077-1-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>