drm/radeon: set correct number of banks for CIK chips in DCE

We don't have the NUM_BANKS parameter, so we have to calculate it
from the other parameters. NUM_BANKS is not constant on CIK.

This fixes 2D tiling for the display engine on CIK.

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