drm/i915: Fix the interlace mode selection for gmch platforms

PIPECONF_INTERLACE_W_FIELD_INDICATION is only meant to be used for sdvo
since it implies a slightly weird vsync shift of htotal/2. For everything
else we should use PIPECONF_INTERLACE_W_SYNC_SHIFT and let the value in
the VSYNCSHIFT register take effect.

The only exception is gen3 simply because VSYNCSHIFT didn't exist yet.
Gen2 doesn't support interlaced modes at all, so we can drop the
explicit gen2 checks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>