drm/i915/fbc: Flatten __intel_fbc_pre_update()

Use an early return to flatten most of __intel_fbc_pre_update().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211124113652.22090-10-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>