Our tessellation control shaders can be dispatched in several modes.

- SINGLE_PATCH (Gen7+) processes a single patch per thread, with each
  channel corresponding to a different patch vertex.  PATCHLIST_N will
  launch (N / 8) threads.  If N is less than 8, some channels will be
  disabled, leaving some untapped hardware capabilities.  Conditionals
  based on gl_InvocationID are non-uniform, which means that they'll
  often have to execute both paths.  However, if there are fewer than
  8 vertices, all invocations will happen within a single thread, so
  barriers can become no-ops, which is nice.  We also burn a maximum
  of 4 registers for ICP handles, so we can compile without regard for
  the value of N.  It also works in all cases.

- DUAL_PATCH mode processes up to two patches at a time, where the first
  four channels come from patch 1, and the second group of four come
  from patch 2.  This tries to provide better EU utilization for small
  patches (N <= 4).  It cannot be used in all cases.

- 8_PATCH mode processes 8 patches at a time, with a thread launched per
  vertex in the patch.  Each channel corresponds to the same vertex, but
  in each of the 8 patches.  This utilizes all channels even for small
  patches.  It also makes conditions on gl_InvocationID uniform, leading
  to proper jumps.  Barriers, unfortunately, become real.  Worse, for
  PATCHLIST_N, the thread payload burns N registers for ICP handles.
  This can burn up to 32 registers, or 1/4 of our register file, for
  URB handles.  For Vulkan (and DX), we know the number of vertices at
  compile time, so we can limit the amount of waste.  In GL, the patch
  dimension is dynamic state, so we either would have to waste all 32
  (not reasonable) or guess (badly) and recompile.  This is unfortunate.
  Because we can only spawn 16 thread instances, we can only use this
  mode for PATCHLIST_16 and smaller.  The rest must use SINGLE_PATCH.

This patch implements the new 8_PATCH TCS mode, but leaves us using
SINGLE_PATCH by default.  A new INTEL_DEBUG=tcs8 flag will switch to
using 8_PATCH mode for testing and benchmarking purposes.  We may
want to consider using 8_PATCH mode in Vulkan in some cases.

The data I've seen shows that 8_PATCH mode can be more efficient in
some cases, but SINGLE_PATCH mode (the one we use today) is faster
in other cases.  Ultimately, the TES matters much more than the TCS
for performance, so the decision may not matter much.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>