drm/i915: reject modes the LPT FDI receiver can't handle

More specifically, the LPT FDI RX only supports 8bpc and a maximum of
2 lanes, so anything above that won't work and should be rejected.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>