drm/tests: Zero initialize fourccs_out

fourccs_out array is not initialized. As the
drm_fb_build_fourcc_list() doesn't necessarily change all the array,
and the test compares all of it, the comparison could fail if the
array is not initialized. Zero initialize the array to fix this.

Fixes: 371e0b18 ("drm/tests: Add KUnit tests for drm_fb_build_fourcc_list()")
Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901-zero-init-fourcc-list-test-v1-1-68bc4cc738c8@riseup.net