backend/drm: remove wlr_drm_crtc.legacy_crtc

We only need it for one thing: gamma size. Moreover, some bits in
the drmModeCrtc will become out-of-date, for instance the current
mode, so let's avoid caching the whole struct and only keep what
we know won't change.

include/backend/drm/drm.h

@@ -41,7 +41,7 @@ struct wlr_drm_crtc {
 	uint32_t gamma_lut;
 
 	// Legacy only
-	drmModeCrtc *legacy_crtc;
+	int legacy_gamma_size;
 
 	struct wlr_drm_plane *primary;
 	struct wlr_drm_plane *cursor;