drm/i915: Rename s/skl_compute_pipe_wm/skl_build_pipe_wm/

When we added atomic watermarks, we added a new display vfunc
'compute_pipe_wm' that is used to compute any pipe-specific watermark
information that we can at atomic check time.  This was a somewhat poor
naming choice since we already had a 'skl_compute_pipe_wm' function that
doesn't quite fit this model --- the existing SKL function is something
that gets used at atomic commit time, after the DDB allocation has been
determined.  Let's rename the existing SKL function to avoid confusion.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-3-git-send-email-matthew.d.roper@intel.com