video: exynos_dp: Enable hotplug interrupts

Enable hotplug interrupts and move the hotplug scheduling into the
interrupt handler. This allows us to introduce a screen at any time
while we're running.

[jg1.han@samsung.com: moved the bit masking of hotplug interrupts]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>