drm/i915: Remove check for !crtc_state in intel_plane_atomic_calc_changes()

smatch spotted that:

	drivers/gpu/drm/i915/intel_display.c:11986
	intel_plane_atomic_calc_changes() warn: variable dereferenced before
	check 'crtc_state' (see line 11972)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1467470166-31717-9-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>