drm/i915: Add new CRC sources

On pre-gen5 and vlv we can't use the pipe source when TV-out or a DP
port is connected to the pipe. Hence we need to expose new CRC
sources.

Also simplify the existing pipe source platform code a bit by
rejecting all unhandled sources by default.

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