drm/i915: Add GEN7_GPGPU_DISPATCHDIMX/Y/Z to the register whitelist

This is required to support glDispatchComputeIndirect for gen7.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>