drm/exynos: Remove "internal" interrupt handling

Remove the "internal" interrupt handling since it's never invoked and
remove "external" reference. This patch removes a bunch of dead code
and clarifies how hotplugging is handled in the HDMI driver.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>