drm/i915: Add GPGPU_THREADS_DISPATCHED to the register whitelist

This will allow us to read the number of dispatched compute threads
for GL_ARB_pipeline_statistics_query.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>