drm/modes: parse_cmdline: Make various char pointers const

We are not supposed to modify the passed in string, make char pointers
used in drm_mode_parse_cmdline_options() const char * where possible.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118155134.30468-2-hdegoede@redhat.com