drm/tests: Test default pitch fallback

Test the default pitch fallback when NULL is passed as the dst_pitch on
the conversion procedures.

Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230814-gsoc-drm-format-test-v2-v3-1-bd3e9f9bc2fb@riseup.net