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

This value is more correct, and matches what we read out in the fastboot
code.  Without this, the watermark code will panic after the first mode
setting activity after a fastboot.

v2: fix up HSW ->clock usage too (Ville)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>