drm/i915: rework IS_*_GT* macros

We can now make use of the intel_device_info.gt field.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170830161208.29221-4-lionel.g.landwerlin@intel.com