drm/radeon: Fix num_banks calculation for SI

The way the tile mode array index was calculated only makes sense for
the CIK specific macrotile mode array. For SI, we need to use one of the
tile mode array indices reserved for displayable surfaces.

This happened to result in correct display most if not all of the time
because most of the SI tiling modes use the same number of banks.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>