drm/i915: simplify pipe checking

This is also handled by i915_reg.h, so just reuse this trick to reduce
universe entropy.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Cyril Brulebois <kibi@debian.org>
Signed-off-by: Keith Packard <keithp@keithp.com>