drm/i915: Preserve the DDI_A_4_LANES bit from the bios

commit bcf53de4e60d9000b82f541d654529e2902a4c2c upstream.

Otherwise the DDI_A_4_LANES bit gets lost and we can't use > 2 lanes
on eDP. This fixes eDP on hsw with > 2 lanes.

Also s/port_reversal/saved_port_bits/ since the current name is
confusing.

Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Zhouping Liu <zliu@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>