drm/modes: Switch to 64bit maths to avoid integer overflow

drm/modes: Switch to 64bit maths to avoid integer overflow

The new >8k CEA modes have dotclocks reaching 5.94 GHz, which
means our clock*1000 will now overflow the 32bit unsigned
integer. Switch to 64bit maths to avoid it.

Cc: stable@vger.kernel.org
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201022194256.30978-1-ville.syrjala@linux.intel.com
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>