drm/tests: Add multi-plane support to conversion_buf_size()

The drm_fb_memcpy() supports multi-plane formats. To fully test it in
the future, add multi-plane support to the conversion_buf_size() helper.

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/20230814-gsoc-drm-format-test-v2-v3-5-bd3e9f9bc2fb@riseup.net