drm/qxl: prevent memory leak

drm/qxl: prevent memory leak

The allocated memory for qdev->dumb_heads should be released
in qxl_destroy_monitors_object before qxl suspend.
otherwise,qxl_create_monitors_object will be called to
reallocate memory for qdev->dumb_heads after qxl resume,
it will cause memory leak.

Signed-off-by: Zongmin Zhou <zhouzongmin@kylinos.cn>
Link: https://lore.kernel.org/r/20230801025309.4049813-1-zhouzongmin@kylinos.cn
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>