drm/i915: Remove the IS_SNB_GT1 define

The last (only?) user of this was removed in:

  commit 2208d655
  Author: Daniel Vetter <daniel.vetter@ffwll.ch>
  Date:   Fri Nov 14 09:25:29 2014 +0100

    drm/i915: drop WaSetupGtModeTdRowDispatch:snb

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>