drm/selftests: reduce stack usage

Putting a large drm_connector object on the stack can lead to warnings
in some configuration, such as:

drivers/gpu/drm/selftests/test-drm_cmdline_parser.c:18:12: error: stack frame size of 1040 bytes in function 'drm_cmdline_test_res' [-Werror,-Wframe-larger-than=]
static int drm_cmdline_test_res(void *ignored)

Since the object is never modified, just declare it as 'static const'
and allow this to be passed down.

Fixes: b7ced389 ("drm/selftests: Add command line parser selftests")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628121712.1928142-1-arnd@arndb.de