drm/radeon/kms: match r6xx/r7xx/evergreen asic_reset with previous asics

Don't reset if the engine isn't busy.  This matches the behavior of
previous asics.  Reseting a non-hung block can lead to a hang.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=33272



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>