gma/gma500: fix a memory disclosure bug due to uninitialized bytes

`best_clock` is an object that may be sent out. Object `clock`
contains uninitialized bytes that are copied to `best_clock`,
which leads to memory disclosure and information leak.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191018042953.31099-1-kjlu@umn.edu