drm/vc4: hdmi: Fixed possible integer overflow

Multiplying ints and saving it in unsigned long long
could lead to integer overflow before being type casted to
unsigned long long.

Addresses-Coverity:  1505113: Unintentional integer overflow.
Signed-off-by: Saud Farooqui <farooqui_saud@hotmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/PA4P189MB1421E63C0FF3EBF234A80AB38BA79@PA4P189MB1421.EURP189.PROD.OUTLOOK.COM

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -1481,7 +1481,7 @@ vc4_hdmi_encoder_compute_mode_clock(const struct drm_display_mode *mode,
 				    unsigned int bpc,
 				    enum vc4_hdmi_output_format fmt)
 {
-	unsigned long long clock = mode->clock * 1000;
+	unsigned long long clock = mode->clock * 1000ULL;
 
 	if (mode->flags & DRM_MODE_FLAG_DBLCLK)
 		clock = clock * 2;