drm/tegra: Handle 64-bit return from drm_crtc_vblank_count()

570e86963a51 ("drm: Widen vblank count to 64-bits [v3]") changed the
return type for drm_crtc_vblank_count() to u64. This could cause
potential problems if the return value is used in arithmetic operations
with a 32-bit reference HW vblank count. Explicitly typecasting this
down to u32 either fixes a potential problem or serves to add clarity in
case the implicit typecasting was already correct.

Cc: Keith Packard <keithp@keithp.com>
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203051302.9974-6-dhinakaran.pandiyan@intel.com

drivers/gpu/drm/tegra/dc.c

@@ -1359,7 +1359,7 @@ static u32 tegra_dc_get_vblank_counter(struct drm_crtc *crtc)
 		return host1x_syncpt_read(dc->syncpt);
 
 	/* fallback to software emulated VBLANK counter */
-	return drm_crtc_vblank_count(&dc->base);
+	return (u32)drm_crtc_vblank_count(&dc->base);
 }
 
 static int tegra_dc_enable_vblank(struct drm_crtc *crtc)