Current code sets alpha mode to global alpha mode and global alpha
value to 0xff which is totaly opaque. That is not needed for two
reasons:

- only one plane is active and thus it can be blended only with
background, which is black,
- it will hinder proper blending when more than one plane is supported

Default mode (0) considers pixel alpha value or 0xff if pixel has
no alpha information. Global alpha value is ignored in this case.

Because of that, just remove the code.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-4-jernej.skrabec@siol.net