drm/i915: We implement WaMiSetContext_Hang

WaMiSetContext_Hang tells us that a MI_NOOP must follow MI_SET_CONTEXT.

The other thing WaMiSetContext_Hang seems to say is that URB_FENCE isn't
allowed to straddle two cachelines. But we don't issue those from the
kernel so we don't care.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>