drm/i915: hw state readout support for pipe_config->fdi_lanes

v2: Introduce some nice #defines for the FDI lane width fields and put
them to good use. Suggested by Ville.

v3: Fixup the mask vs. shift copy&pasta fail Imre Deak spotted, and
use the shift #define also in the mask.

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