drm/radeon: update wait_for_vblank for evergreen+

commit 10257a6d upstream.

Properly wait for the next vblank region.  The previous
code didn't always wait long enough depending on the timing.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>