msm: mdp: Move wfd state signalling into mdp driver

Previously the WFD driver was reponsible for signalling the
online/offline events to userspace listeners.  This led to redundant
notifications due to the manner in which the WFD driver is used. The
signalling is better purposed when done from MDP driver.

Change-Id: I2248de84d57504c2fa9f26e1bfdf0e0d4a02257a
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>