drm/i915/skl: Do not allow scaling when crtc is disabled.

This fixes a warning when the crtc is turned off. In that case fb
will be NULL, and crtc_clock will be 0. Because the crtc is no longer
active this is not a bug, and shouldn't trigger the WARN_ON.

Also remove handling a null crtc_state, with all transitional helpers
gone this can no longer happen.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448360945-5723-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>