tests: drivers: display: fix filtering

Test was marked build only with filters in each scenario looking for
sdl-dc which is only available on native_sim, so cut the chase and use
platform_only to narrow things down to native_sim directly instead of
building the world to get information we already know.

reduces build/run time from 78s to 17s on invocation of twister with
default options, saves a ton more when running twister with --all.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>