drm/radeon: add proper support for RADEON_VM_BLOCK_SIZE v2

This patch makes it possible to decide how many address
bits are spend on the page directory vs the page tables.

v2: remove unintended change

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>