msm: mdss: Handle fb_release_all before cont-splash handoff

By default android framework sends a NULL commit initially,
before sending any frames. When continuous splash is enabled,
driver skips handling this NULL commit to avoid blank frames
during bootup. However, it has to be processed if it is due
to fb_release_all, as pipes need to be unstaged before going
to suspend state.

Change-Id: Id40ddd85ea59125f177fb7054554bccf35546fbd
Signed-off-by: Shalini Krishnamoorthi <shakri@codeaurora.org>