drm/i915/skl: Assume no scaling is available when things are not as expected

drm/i915/skl: Assume no scaling is available when things are not as expected

Cdclk < crtc_clock is not allowed and suggests a different problem
elsewhere in the code.

It is more robust and safe to assume no scaling is possible in
this case with no other downsides since it will also WARN_ON_ONCE
so that this definitely gets noticed.

Call it an assert to help new platform bring-up in simulation.

v2: Better commit msg and use WARN_ON_ONCE to signify the unexpectedness.

v3: Move zero crtc_clock check under the warn. (Ville)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>