drm/i915: Add VECS semaphore bits

Like the other rings, the VECS supports semaphores. The semaphore stuff
is a bit wonky so this patch on it's own should be nice for review.

This patch should have no functional impact.

v2: Fix the English parts of clarification (again, register names were
right, text was reversed) (Damien)
Restore the still valid invariant. (Damien)
The bsd semaphore register should be MI_SEMAPHORE_SYNC_VVE (Damien)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>