drm/i915: correctly program the VSYNCSHIFT register

The hw seems to use this to correctly insert the required delay
before/after an even/odd interlaced field. This might also explain
why we need to substract 1 half-line from vtotal - if the hw just
adds the delay programmend in VSYNCSHIFT the total frame time would be
about that too long.

These registers seems to only exist on gen4 and later. For paranoia
also program it to 0 for progressive modes, but according to
documentation the hw should just ignore it in this case.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Alfonso Fiore <alfonso.fiore@gmail.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>