drm/i915: Check the clock from adjusted mode in intel_crtc_active()

The clock in crtc->mode doesn't necessarily mean anything. Let's look
at the clock in adjusted_mode instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>