drm/i915: Add a small adjustment to the pixel counter on interlaced modes

In interlaced modes, the pixel counter counts all pixels,
so one field will have htotal more pixels. In order to avoid
the reported position from jumping backwards when the pixel
counter is beyond the length of the shorter field, just
clamp the position the length of the shorter field. This
matches how the scanline counter based position works since
the scanline counter doesn't count the two half lines.

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>