drm/mst: Fix up u64 division

Change rem_nsec to u32 since that's what do_div returns, this avoids the
u64 divide in the drm_print args.

Changes in v2:
- Instead of doing do_div in drm_print, make rem_nsec u32 (Ville)

Link to v1: https://patchwork.freedesktop.org/patch/msgid/20191106173622.15573-1-sean@poorly.run

Fixes: 12a280c7

 ("drm/dp_mst: Add topology ref history tracking for debugging")
Cc: Juston Li <juston.li@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <sean@poorly.run>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106194121.164458-1-sean@poorly.run