mesa: Add missing _mm_mfence() before streaming loads.

mesa: Add missing _mm_mfence() before streaming loads.

According to the Intel Software Development Manual (Volume 1: Basic
Architecture, 12.10.3 Streaming Load Hint Instruction):

   Streaming loads may be weakly ordered and may appear to software to
   execute out of order with respect to other memory operations.
   Software must explicitly use fences (e.g. MFENCE) if it needs to
   preserve order among streaming loads or between streaming loads and
   other memory operations.

That is, a memory fence is needed to preserve the order between the GPU
writing the buffer and the streaming loads reading it back.

Reported-by: Joseph Nuzman <joseph.nuzman@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>