drm/i915/fbc: Extract snb_fbc_program_fence()

We have two identical copies of the snb+ system agent
CPU fence programming code. Extract into a helper.

Also there's no real point in insisting that we
program 0 into DPFC_CPU_FENCE_OFFSET when the fence is
disabled. So just always stick the computed Y offset there
whether or not the fence is actually used or not.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-2-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>