msm/sde/rotator: avoid dropping alpha channel fetch for rotator sspp

Enable alpha channel fetch for UI layers as constant alpha
information is not passed to rotator driver.

This will help in fixing artifacts where in the alpha value is
not linear in the layer.

Change-Id: Ia8ea67ee2af4664d1be56d38c69af49a7a3261ac
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>