drm/bridge/sii8620: simplify hardware reset procedure

There is no need to flip reset pin twice. Also delays can be changed to
values present in vendor's code.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Maciej Purski <m.purski@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608060457.18357-1-a.hajda@samsung.com