hdmi_setup_fake_chmap() is supposed to set the reported channel map when
the channel map is not specified by the user.

However, the function indexes channel_allocations[] with a wrong value
and extracts the wrong nibble from hdmi_channel_mapping[], causing wrong
channel maps to be shown.

Fix those issues.

Tested on Intel HDMI to correctly generate various channel maps, for
example 3,4,14,15,7,8,5,6 (instead of incorrect 3,4,8,7,5,6,14,0) for
standard 7.1 channel audio. (Note that the side and rear channels are
reported as RL/RR and RLC/RRC, respectively, as per the CEA-861
standard, instead of the more traditional SL/SR and RL/RR.)

Note that this only fixes the layouts that only contain traditional 7.1
speakers (2.0, 2.1, 4.0, 5.1, 7.1, etc.). E.g. the rear center of 6.1
is still being shown wrongly due to an issue with from_cea_slot()
which will be fixed in a later patch.
Signed-off-by: NAnssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>