drm/i915: Preserve the DDI_A_4_LANES bit from the bios

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>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>