msm: mdss: Remove union for output buffer and retire fence

In order to provide more reusable code, and prevent unnecessary
debugging removing union as memory saving is insignificant.

Change-Id: I5da68c64c0bf09157738fbec1feb6adde5af42a2
[veeras@codeaurora.org: Resolve merge conflict in msm_mdp_ext.h by
retaining the version in this file]
Signed-off-by: Terence Hampson <thampson@codeaurora.org>
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>