[Intel-gfx] (no subject)

[Dave Gordon]

Updated version split into two. The first tidies up the _ring_prepare()
functions and removes the corner case where we might have had to wait
twice; the second is a temporary workaround to solve a kernel OOPS that
can occur if logical_ring_begin is called while the ringbuffer is not
mapped because there's no current request.

The latter will be superseded by the Anti-OLR patch series currently
in review. But this helps with GuC submission, which is better than
the execlist path at exposing the problematic case :(