i915: Fix up intelInitScreen2 for DRI3

i915: Fix up intelInitScreen2 for DRI3

Commit 442442026eb updated both i915 and i965 for DRI3 support,
but one check in intelInitScreen2 was missed for i915 causing crashes
when trying to use i915 with DRI3.

So fix that up.

Reported-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
References: https://bugzilla.redhat.com/show_bug.cgi?id=1115323
References: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=754297
Tested-by: František Zatloukal <Zatloukal.Frantisek@gmail.com>
Tested-by: Dirk Griesbach <spamthis@freenet.de>
Signed-off-by: Adel Gadllah <adel.gadllah@gmail.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>