drm/i915/fbc: convert intel_fbc.c to use INTEL_GEN()

Because it's shorter, easier to read, newer and cooler. And I don't
think anybody else has pending FBC patches right now, so the conflicts
should be minimal.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478883461-20201-8-git-send-email-paulo.r.zanoni@intel.com