drm/omap: decrease min width & height

mode_config's min_width and min_height are both set to 32, which is
overly restrictive.

The real limits depend on whether we're configuring a crtc or a plane,
but a limit of 8x2 is safe for both cases.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>