Decreased conditions for amix_out_cur to amix_out assignment.

[esden]

The alphamixer_out <= alphamixer_out_cur can be done outside of the
state machine as it is an output dependent on the vid_wen bit. It very
slightly improves the timing for clk48m from 49.36 MHz to 49.74 MHz.

I will be adding another combinatorial step in that output path as part
of the workshop. In that case this change make a huge timing difference
fail vs pass. Having this minor change will make the workshop bit less
convoluted and easier to follow.

Of course I might be missing something and this is a terrible idea. Please let me know. :D

[Spritetm]

Sorry for taking so long to merge this... I think I recall there may be an edge case here somewhere as the entire pixel pipeline is pretty pipelined but sometimes needs to stop because copper or DMA wait, but I haven't found the time to properly look into this yet.