drm/i915/fbc: Make fence_id optional for i965gm

drm/i915/fbc: Make fence_id optional for i965gm

i965gm no longer needs the fence for scanout so we should be
do what we do for ctg+ and only configure a fence for FBC
when we have one.

In theory this should do nothing atm on account of
intel_fbc_can_activate() requiring the fence, but since
we do this for g4x+ let's do it for i965gm as well. We
may want to relax the requirements at some point and allow
FBC without a fence.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-9-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>