drm/i915: use the mode->htotal to calculate linetime watermarks

... instead of mode->crtc_display. The spec says "pipe horizontal
total number of pixels" and the "Haswell Watermark Calculator" tool
uses the "Pipe H Total" instead of "Pipe H Src" as the value.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>