drm/i915: Fix gen2 and hsw+ scanline counter

On gen2 the scanline counter behaves a bit differently from the
later generations. Instead of adding one to the raw scanline
counter value, we must subtract one.

On HSW/BDW the scanline counter requires a +2 adjustment on HDMI
outputs. DP outputs on the on the other require the typical +1
adjustment.

As the fixup we must apply to the hardware scanline counter
depends on several factors, compute the desired offset at modeset
time and tuck it away for when it's needed.

v2: Clarify HSW+ situation

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;">
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78997


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>