drm/i915: Add a workaround for HSW scanline counter weirdness

drm/i915: Add a workaround for HSW scanline counter weirdness

On HSW the scanline counter seems to behave differently depending on
the output type. eDP on port A does what you would expect an the normal
+1 fixup is sufficient to cover it. But on HDMI outputs we seem to need
a +2 fixup. Just assume we always need the +2 fixup and accept the
slight inaccuracy on eDP.

This fixes a regression introduced in:
 commit 1a91ea92ce40301a9774e7a987fee8801ccd8727
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Mon Oct 28 21:22:52 2013 +0200

    drm/radeon: Move the early vblank IRQ fixup to radeon_get_crtc_scanoutpos()

That commit removed the heuristic that tried to fix up the timestamps
for vblank interrupts that fire a bit too early. Since then the vblank
timestamp code would treat some vblank interrupts as spurious since the
scanline counter would indicate that vblank_start wasn't reached yet.
That in turn lead to incorrect vblank event sequence numbers being
reported to userspace, which lead to unsteady framerate in applications
such as XBMC which uses them for timing purposes.

v2: Remember to call ilk_pipe_in_vblank_locked() on HSW too (Mika)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75725
Tested-by: bugzilla1@gmx.com
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>