Some options which can be specified on the commandline, such as
margin_right=..., margin_left=..., etc. are applied not only to the
specified mode, but to all modes. As such it would be nice if the user
can simply say e.g.
video=HDMI-1:margin_right=14,margin_left=24,margin_bottom=36,margin_top=42

This commit refactors drm_mode_parse_command_line_for_connector() to
add support for this, and as a nice side effect also cleans up the
function a bit.
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118155134.30468-8-hdegoede@redhat.com