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.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>