i915: workaround multiple intelFenceExtension definitions

i915: workaround multiple intelFenceExtension definitions

Due to conflicting symbol names (between i915 and i965) in the
megadriver, we use a set of defines in i915/intel_screen.h.

With a recent commit we've introduced a symbol intelFenceExtension which
has different implementation for each driver, yet we forgot to add the
define.

Fixes: d11515ff1b3 ("i915/sync: Implement DRI2_Fence extension")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98264
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>