drm/amd/dc/dce120: Fix audio register mapping, stop triggering KASAN

There's been a very long running bug that seems to have been neglected for
a while, where amdgpu consistently triggers a KASAN error at start:

  BUG: KASAN: global-out-of-bounds in read_indirect_azalia_reg+0x1d4/0x2a0 [amdgpu]
  Read of size 4 at addr ffffffffc2274b28 by task modprobe/1889

After digging through amd's rather creative method for accessing registers,
I eventually discovered the problem likely has to do with the fact that on
my dce120 GPU there are supposedly 7 sets of audio registers. But we only
define a register mapping for 6 sets.

So, fix this and fix the KASAN warning finally.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>