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>