ALSA: emu10k1: polish audigy GPR allocation

- Pull ahead all fixed allocations, so we don't rely on the semi-
  dynamic ones not crossing the arbitrarily determined limit
- Use an enum for the fixed allocations
- Stop arbitrarily wasting registers on unexplained "reservations"
- Don't reserve two regs for the master volume control - it's mono

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230514170323.3408834-2-oswald.buddenhagen@gmx.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>