drm/meson: Use 1000ULL when operating with mode->clock

drm/meson: Use 1000ULL when operating with mode->clock

Coverity scan reported the usage of "mode->clock * 1000" may lead to
integer overflow. Use "1000ULL" instead of "1000"
when utilizing it to avoid potential integer overflow issue.

Link: https://scan5.scan.coverity.com/#/project-view/10074/10063?selectedIssue=1646759
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Fixes: 1017560164b6 ("drm/meson: use unsigned long long / Hz for frequency types")
Link: https://lore.kernel.org/r/20250505184338.678540-1-richard120310@gmail.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>