Add atomic gamma lut size fetching

Legacy gamma lut size now uses the new legacy_crtc member of
wlr_drm_crtc. This was Previously doen using old_crtc in
wlr_drm_connector, but since this refers to the crtc that was connected to
the ouput, this could give the wrong result.

include/backend/drm/drm.h

@@ -45,6 +45,9 @@ struct wlr_drm_crtc {
 	uint32_t gamma_lut;
 	drmModeAtomicReq *atomic;
 
+	// Legacy only
+	drmModeCrtc *legacy_crtc;
+
 	union {
 		struct {
 			struct wlr_drm_plane *overlay;