ALSA: emu10k1: skip needless setting of some voice registers

Many registers are meaningless for stereo slaves and the extra voices.
This patch cleans up these unnecessary register writes.

snd_emu10k1_playback_{trigger,stop}_voice() is not called for stereo
slaves any more.

snd_emu10k1_playback_prepare_voice() is renamed to
snd_emu10k1_playback_unmute_voice(), as this better reflects its
remaining function. It's not called for the extra voices any more.

Accordingly, snd_emu10k1_playback_mute_voice() is factored out from
snd_emu10k1_playback_stop_voice(), and is called selectively as well.

This doesn't add conditionals which would avoid initializing
sub-registers, as that wouldn't pull its weight.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230516093612.3536451-6-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/emu10k1.h

@@ -404,6 +404,14 @@ SUB_REG(HCFG, LOCKTANKCACHE,	0x00000004)	/* 1 = Cancel bustmaster accesses to ta
 //   distortion), the modulation engine sets the target registers, towards
 //   which the current registers "swerve" gradually.
 
+// For the odd channel in a stereo pair, these registers are meaningless:
+//   CPF_STEREO, CPF_CURRENTPITCH, PTRX_PITCHTARGET, CCR_CACHEINVALIDSIZE,
+//   PSST_LOOPSTARTADDR, DSL_LOOPENDADDR, CCCA_CURRADDR
+// The somewhat non-obviously still meaningful ones are:
+//   CPF_STOP, CPF_FRACADDRESS, CCR_READADDRESS (!),
+//   CCCA_INTERPROM, CCCA_8BITSELECT (!)
+// (The envelope engine is ignored here, as stereo matters only for verbatim playback.)
+
 #define CPF			0x00		/* Current pitch and fraction register			*/
 SUB_REG(CPF, CURRENTPITCH,	0xffff0000)	/* Current pitch (linear, 0x4000 == unity pitch shift) 	*/
 #define CPF_STEREO_MASK		0x00008000	/* 1 = Even channel interleave, odd channel locked	*/