drm/vgem: add missing mutex_destroy

vgem_fence_open() instantiates a mutex for a particular fence
instance, but never destroys it by calling mutex_destroy() in
vgem_fence_close().

So, add the missing mutex_destroy() to guarantee proper resource
destruction.

Fixes: 40777984

 ("drm/vgem: Attach sw fences to exported vGEM dma-buf (ioctl)")
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230202125517.427976-1-mcanal@igalia.com