drm/i915: Improve gen3/4 frame counter

Currently the logic to fix up the frame counter on gen3/4 assumes that
start of vblank occurs at vblank_start*htotal pixels, when in fact
it occurs htotal-hsync_start pixels earlier. Apply the appropriate
adjustment to make the frame counter more accurate.

Also fix the vblank start position for interlaced display modes.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: "Akash Goel <akash.goels@gmail.com&gt;">
Reviewed-by: "Sourab Gupta <sourabgupta@gmail.com&gt;">
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>