msm: mdss: send fb events for both pdata for split DSI cases

FB events are sent only for first pdata and not for the next
in split DSI cases. This is handled within each event separately.
Fix the event handler to send events for both pdata separately
in split DSI cases and avoid handling it within individual events.

Change-Id: I70edc88f0958af3a807eecf0211dc2d8cd26301c
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>