drm/radeon: Always flush VM again on < CIK

Not doing this causes piglit hangs[0] on my Cape Verde card. No issues on
Bonaire and Kaveri though.

[0] Same symptoms as those fixed on CIK by 'drm/radeon: set VM base addr
using the PFP v2'.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>