i915g: Use the color swizzle to reshuffle the blend const color.

i915g: Use the color swizzle to reshuffle the blend const color.

If we're remapping RGBA from the shader into dst channels, we also need to
remap the blend constant color.  The previous static mapping only worked
for ARGB cbufs, but we get ABGR in surfaceless deqp.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11512>