drm/msm: hangcheck harder

If gpu locks up with the rptr shortly beyond the wrap-around point in
the ringbuffer, because the rptr was not reset (but wptr is, by virtue
of resetting rb->cur), we could end up in a scenario where we think
there is not enough space in the ringbuffer for the next cmds.  And
since the CP won't reset rptr until after processing an IB, this leaves
things in a sort of deadlock.

So reset rptr too.  And a bit more spiffing up of hangcheck to make
things easier to debug.

Signed-off-by: Rob Clark <robdclark@gmail.com>