drm/amdgpu: fix channel index mapping for SIENNA_CICHLID

Pmfw read ecc info registers in the following order,
     umc0: ch_inst 0, 1, 2 ... 7
     umc1: ch_inst 0, 1, 2 ... 7
The position of the register value stored in eccinfo
table is calculated according to the below formula,
     channel_index = umc_inst * channel_in_umc + ch_inst
Driver directly use the index of eccinfo table array
as channel index, it's not correct, driver needs convert
eccinfo table array index to channel index according to
channel_idx_tbl.

Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>