drm/exynos: Debounce gpio hotplug interrupts

drm/exynos: Debounce gpio hotplug interrupts

Debounce the gpio (external) hotplug interrupts. This patch debounces
hotplug interrupts generated while the HDMI block is off. The reason
this is needed is that we get multiple (5) interrupts every time a
monitor is inserted which causes us to needlessly enable and disable the
IP block.

BUG=chromium:220033
TEST=Tested using an HDMI analyzer. Many hotplugs without any 20s freeze

Change-Id: I9c5d61364790f4110fc758a93fabac48b646b3fa
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49349
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>