Fix HDMI unblank behavior

Two issues:

(1) We were announcing the hotplug event before we were ready to
handle blank/unblank events, so we were losing the initial unblank
that power manager sends us when HDMI is first plugged in.  This
left the display blank until you toggled the device power off/on.

(2) We were retaining fbTargetHandle for HDMI after the display was
disconnected.  The value didn't get updated when HDMI was reconnected
because the display was blank, so we didn't go through that code
path.  So, when HDMI was re-connected, we passed stale data into
the HWC.

Bug 7323938

Change-Id: I2335d24fd7b0f00bb23fc63aa7bcf44cb8857c73