drm/i915: add IS_GEN7 macro to cover Ivy Bridge and later

Note: IS_GEN* are for render related checks.  Display and other checks
should use IS_MOBILE, IS_$CHIPSET or test for specific features.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>