drm/msm/dp: add support for custom hotplug event in DP driver

Add support to send custom hotplug notifications from display
port driver. This would avoid duplication of code between DRM
upstream driver/MSM DRM driver and also enables to add more
information as part of the uevent string like bpp/video test
pattern information.

CRs-Fixed: 2093830
Change-Id: I186410221c7f642fbea7bd47a838769cfeaee8a1
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>