drm/i915: export size_is_valid() from __intel_fbc_update()

Make the giant function a little less giant.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add pipe_ prefix as suggested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>