drm/amdgpu/gmc10: properly set BANK_SELECT and FRAGMENT_SIZE

These were not aligned for optimal performance for GPUVM.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tianci Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>