drm: Shrink drm_display_mode timings

Store the timings (apart from the clock) as u16. The uapi mode
struct already uses u16 for everything so using something bigger
internally doesn't really help us.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-9-ville.syrjala@linux.intel.com