drm: Use u64 for intermediate dotclock calculations

We have reached the era where monitor bandwidths now exceed 31bits in
frequency calculations, though as we stored them in kHz units we are
safe from overflow in the modelines for some time.

[   48.723720] UBSAN: Undefined behaviour in ../drivers/gpu/drm/drm_modes.c:325:49
[   48.726943] signed integer overflow:
[   48.728503] 2240 * 1000000 cannot be represented in type 'int'

Reported-by: Martin Liška <marxin.liska@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98372


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161021141540.26837-1-chris@chris-wilson.co.uk