drm/i915: Polish the skl+ plane keyval/msk/max register setup

drm/i915: Polish the skl+ plane keyval/msk/max register setup

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.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>