drm: Avoid printing negative values for unsigned variables.

It was really strange to see negative vblank seqs on debug
messages. It is rare to have that big number, but when it
happens it is confusing and misleading.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470243226-2750-1-git-send-email-rodrigo.vivi@intel.com