drm/tegra: dc: Remove set but unused variable 'state'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/tegra/dc.c: In function ‘tegra_crtc_calculate_memory_bandwidth’:
 drivers/gpu/drm/tegra/dc.c:2384:38: warning: variable ‘old_state’ set but not used [-Wunused-but-set-variable]

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/dc.c

@@ -2381,7 +2381,6 @@ static int tegra_crtc_calculate_memory_bandwidth(struct drm_crtc *crtc,
 	const struct tegra_plane_state *tegra_state;
 	const struct drm_plane_state *plane_state;
 	struct tegra_dc *dc = to_tegra_dc(crtc);
-	const struct drm_crtc_state *old_state;
 	struct drm_crtc_state *new_state;
 	struct tegra_plane *tegra;
 	struct drm_plane *plane;
@@ -2396,7 +2395,6 @@ static int tegra_crtc_calculate_memory_bandwidth(struct drm_crtc *crtc,
 		return 0;
 
 	new_state = drm_atomic_get_new_crtc_state(state, crtc);
-	old_state = drm_atomic_get_old_crtc_state(state, crtc);
 
 	/*
 	 * For overlapping planes pixel's data is fetched for each plane at