drm/i915: Add intel_atomic_get_existing_crtc_state function

This patch Adds a function to extract intel_crtc_state from the
atomic_state, if not available it returns NULL.

v2 (from Paulo):
 - Fix white space problem detected by checkpatch.

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161201154940.24446-7-mahesh1.kumar@intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -1809,6 +1809,20 @@ intel_atomic_get_crtc_state(struct drm_atomic_state *state,
 	return to_intel_crtc_state(crtc_state);
 }
 
+static inline struct intel_crtc_state *
+intel_atomic_get_existing_crtc_state(struct drm_atomic_state *state,
+				     struct intel_crtc *crtc)
+{
+	struct drm_crtc_state *crtc_state;
+
+	crtc_state = drm_atomic_get_existing_crtc_state(state, &crtc->base);
+
+	if (crtc_state)
+		return to_intel_crtc_state(crtc_state);
+	else
+		return NULL;
+}
+
 static inline struct intel_plane_state *
 intel_atomic_get_existing_plane_state(struct drm_atomic_state *state,
 				      struct intel_plane *plane)