drm: Make mode->flags u32

The mode flags are direclty exposed in the uapi as u32. Use the
same size type to store them internally.

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/20200428171940.19552-8-ville.syrjala@linux.intel.com