drm/i915: Add a simple atomic crtc check function, v2.

Move the check for encoder cloning here.

Changes since v1:
- Remove was/is crtc_disabled. (mattrope)
- Rename function to intel_crtc_atomic_check. (mattrope)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>