drm/radeon: adjust default radeon_vm_block_size v2

v2: rebase on vm_size scale change. Adjust vm_size default to 8,
    Better handle the default and smaller values.

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

diff --cc drivers/gpu/drm/radeon/radeon_device.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_drv.c
Simple merge