drm/i915: Adjust CRC capture for pre-gen5/vlv

Should work down to gen2. The #defines for the interrupt sources are
already there in PIPESTAT and are the same on all gmch platforms for
gen2 up to vlv.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>