drm/i915: Convert hsw_compute_linetime_wm to use in-flight state

drm/i915: Convert hsw_compute_linetime_wm to use in-flight state

When watermark calculation was moved up to the atomic check phase, the
code was updated to calculate based on in-flight atomic state rather
than already-committed state.  However the hsw_compute_linetime_wm()
didn't get updated and continued to pull values out of the
currently-committed CRTC state.  On platforms that call this function
(HSW/BDW only), this will cause problems when we go to enable the CRTC
since we'll pull the current mode (off) rather than the mode we're
calculating for and wind up with a divide by zero error.

This was an oversight in commit:

        commit a28170f3389f4e42db95e595b0d86384a79de696
        Author: Matt Roper <matthew.d.roper@intel.com>
        Date:   Thu Sep 24 15:53:16 2015 -0700

            drm/i915: Calculate ILK-style watermarks during atomic check (v3)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449171462-30763-5-git-send-email-matthew.d.roper@intel.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>