drm: Shrink {width,height}_mm to u16

Instead of supporting ~2000km wide displayes let's limit ourselves
to ~65m. That seems plenty big enough to me.

Even with EDID_QUIRK_DETAILED_IN_CM EDIDs seem to be limited to
10*0xfff which fits into the 16 bits.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-6-ville.syrjala@linux.intel.com