msm: mdss: fix staging VIG3 and RGB3 pipes at stage6

Starting MDSS 1.5, display controller can blend 7 layers excluding
base layer. If a pipe is staged on stage6, it requires use of extension
register. Current logic has a bug where if VIG3 of RGB3 pipes are on
stage6 then configuration is incorrect and leads to bad HW behaviour.
Fix this by correcting the staging logic.

Change-Id: I4f34783a9bd8ae5e0898bcf25755cf687f195211
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>