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>