drm/i915: Remove superfluous NULL check

to_intel_crtc() always returns a non-NULL pointer.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1448986198-3488-2-git-send-email-tiwai@suse.de