drm/vc4: hdmi: Set default state margin at reset

Now that the TV margins are properly parsed and filled into
drm_cmdline_mode, we just need to initialise the first state at reset to
get those values and start using them.

Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/44e24172e300be6a41578517021ef6a6e90ed682.1560783090.git-series.maxime.ripard@bootlin.com