Due to the constant alpha we're going to have to program two of
the the tree keying registers anyway, so might as well always
program all three.

And parametrize the plane constant alpha define while at it.

v2: Rebase due to input CSC
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107184138.31359-1-ville.syrjala@linux.intel.comReviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>