drm/amdgpu: increase fragmentation size for Vega10 v2

The fragment bits work differently for Vega10 compared to previous generations.

Increase the fragment size to 2MB for now to better handle that.

v2: handle the hardware setup as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-tested-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>