drm/vc4: add extcon hdmi connection uevent

drm/vc4: add extcon hdmi connection uevent

Add extcon hdmi connection and disconnection uevent when extcon
module is enabled.

The vc4_hdmi detection is done by polling way, so extcon uevent
for connection is a bit slow after changing real hdmi cable state.

Change-Id: I962f7a39b7a3344f9793e436ef28c36b123571a8
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
 Conflicts:
	drivers/gpu/drm/vc4/vc4_hdmi.h
	drivers/gpu/drm/vc4/vc4_hdmi.c

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>