drm/modes: Parse overscan properties

Properly configuring the overscan properties might be needed for the
initial setup of the framebuffer for display that still have overscan.
Let's allow for more properties on the kernel command line to setup each
margin.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e481f1628e3768ca49226ec2115cfa4dfcbd5e4c.1560783090.git-series.maxime.ripard@bootlin.com