drm/i915/skl+: WM calculation don't require height

height of plane was require to swap width/height in case of 90/270
rotation. Now src structure contains already swapped values, So we
don't have to calculate height of the plane.

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170705143154.32132-6-mahesh1.kumar@intel.com