drm/i915/skl: Correct other-pipe watermark update condition check (v2)

If ddb allocation for planes in current CRTC is changed, that doesn't
lead to ddb allocation change for other CRTCs, because our DDB allocation
is not dynamic according to plane parameters, ddb is allocated according
to number of CRTC enabled, & divided equally among CTRC's.

In current condition check during Watermark calculation, if number of
plane/ddb allocation changes for current CRTC, Watermark for other pipes
are recalculated. But there is no change in DDB allocation of other pipe
so watermark is also not changed, This leads to warning messages.
WARN_ON(!wm_changed)

This patch corrects this and check if DDB allocation for pipes is changed,
then only recalculate watermarks.

v2 (by Matt): Rebased to latest -nightly and fixed a typo

Signed-off-by: Kumar, Mahesh <mahesh1.kumar@intel.com>
Reviewed-by(v1): Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>