CHROMIUM: drm/rockchip: Clear interrupt status bits before enabling

CHROMIUM: drm/rockchip: Clear interrupt status bits before enabling

The enable register only masks the raw status bits to signal CPU
interrupt only for enabled interrupts. The status bits are activated
regardless of the enable register. This means that we might have an old
interrupt event queued, which we are not interested in. To avoid getting
a spurious interrupt signalled, we have to clear the old bit before we
update the enable register.

BUG=chrome-os-partner:56378
BUG=chrome-os-partner:56580
TEST=while true; do backlight_dbus_tool --set --percent=0 && sleep 8 && backlight_dbus_tool --set --percent=100 && sleep 3 ; done

Change-Id: I1b8286097f3ba102663ee3a7566fd96492e12d26
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/382973
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>