drm/i915/tgl: Add IS_TGL_REVID

We are going to need this macro on limiting
the workaround scope.

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015154449.10338-1-mika.kuoppala@linux.intel.com