msm: mdss: set writeback RGBX8888 output alpha to opaque

Even though the alpha channel on RGBX format should be ignored,
even if the pixel is picked up it should still have full alpha
value so that all pixels are shown. For this reason set the
alpha value to 0xFF.

Change-Id: I244a827b5b0ea228d2b2600dba0c3a1c6d8f2392
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>