drm/i810: make i810_flush_queue() return void

This function always return '0' and no callers use the return value. So
make it a void function.

This eliminates the following coccicheck warning:

drivers/gpu/drm/i810/i810_dma.c:860:8-11: Unneeded variable: "ret".
Return "0" on line 885

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910140610.1191578-1-yanaijie@huawei.com