drm/i915: Wire up gen2 CRC support

Really simple, and we don't even have working frame numbers.

v2: Actually enable it ...

v3: Review from Ville:
- Unconditionally enable the border in the CRC checksum for
  consistency with gen3+.
- Handle the "none" source to be able to disable the CRC machinery
  again.

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