Revert: 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: I2db531b1f6fb3035831195546645d58718cff87a
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org> (reverted from commit 33a412e6b556de14c03a91013085c0560437d5f5)