drm/i915: use crtc_htotal when calculating ilk watermarks

This was introduced in:

commit 7c4a395f
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Oct 9 19:17:56 2013 +0300

    drm/i915: Don't re-compute pipe watermarks except for the affected pipe

and I missed fixing it in:

commit fec8cba3
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Wed Nov 27 11:10:26 2013 -0800

    drm/i915: use crtc_htotal in watermark calculations to match fastboot v2

It's needed for ILK+ platforms to fastboot without crashing on a divide
by 0 after a DPMS on action.

Note: Ville mentioned in his review that this confusion seems to go
down to the original introduction of this code in

commit 801bcfff
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri May 31 10:08:35 2013 -0300

    drm/i915: properly set HSW WM_PIPE registers

So it seems to have been missed both in the fastboot patch and in the
3d mode suppport (where only crtc_htotal reflects the real pipe
width).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add note based on Ville's review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>