drm: msm: add support for a custom hotplug event

Add support for a custom hotplug event that is triggered when the
status of the connector has changed. The event string of the
custom event will be in the following format:
	"name=<connector name> status=<connector status>"
For example, when the DisplayPort connector has its status
updated to connected then the event string that is used as part
of the hotplug event will be as follows:
	"name=DP-1 status=connected"
If the connector name is not known or is invalid at the time that
the event is triggered then the name will be reported as "unknown".

Change-Id: I5d6164d1e8e651cb05527733d68fa86fefb9e6ce
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>