drm_hwcomposer: Fix segfault after Unplug event for disconnected displays

On Android 10, after pressing the power button to turn off the display,
hwcomposer send a "hotplug" event to turn off the display. The power
button could be pressed again to wake up the display, but on some
displays, after pressing the power button, hwcomposer send a unplug
event when the display is still off, after which a segmentation fault
occurs in SurfaceFlinger.

This patch added a display connection check and does not call the
HandleDisplayHotplug() function, which dispatches an hotplug event when
the display is disabled.

Signed-off-by: Maksym Prymierov <maksym.prymierov@globallogic.com>
Change-Id: Ia6f92d5be026a200055765524863e92b0250a7ab
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