drm/radeon/kms: move blit initialization after we disabled VGA

VGA might be overwritting VRAM and corrupt our blit shader leading
to corruption, it likely won't happen if you load fbcon right after
radeon. Thanks to Shawn Starr and Andre Maasikas for tracking down
this issue.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>