msm: mdss: only assign mfd pointer in blank event

Currently mfd pointer is assigned in coming fb_event_callback,
which is possibly invoked in WFD case too. This should be
avoided since DSI check should only apply to primary display.

CRs-fixed: 639687
Signed-off-by: Ray Zhang <rayz@codeaurora.org>
Change-Id: I4592c62349dd7d34082b3e73f5cbb3b49e80dc9b