drm/i915: Reuse normal state readout for LVDS/DVO fixed mode

Reuse the normal state readout code to get the fixed mode for LVDS/DVO
encoders. This removes some partially duplicated state readout code
from LVDS/DVO encoders. The duplicated code wasn't actually even
populating the negative h/vsync flags, leading to possible state checker
complaints. The normal readout code populates that stuff fully.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171009161951.22420-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>