drm/i915: Use drm_encoder_crtc_ok()

The intel_encoder_crtc_ok() is a duplicate of the drm_encoder_crtc_ok()
function that used to be only available in the DRM CRTC helpers. It has
recently been moved to the core, so the duplicate can now be dropped.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>