radeonsi: Fix calculation of number of banks for SI

radeonsi: Fix calculation of number of banks for SI

The way cik_num_banks() was calculating the index only makes sense for
the CIK specific macrotile mode array. For SI, we need to use the tile
mode index directly.

This happened to work most of the time because most of the SI tiling
modes use the same number of banks.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>