drm/i915/skl+: Perform wm level calculations in separate function

Instead of iterating over planes & wm levels in a single function use
skl_compute_wm_level function to interate over WM levels.
Change name of function to skl_compute_wm_levels (Matt).

These changes are to clean-up WM code & will help in making only new
ddb algorithm related changes in later patch in series.

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517115831.13830-10-mahesh1.kumar@intel.com