drm/format-helper: Use drm_format_info_min_pitch() in tests helper

There's a nice macro to calculate the destination pitch that already takes
into account sub-byte pixel formats. Use that instead of open coding it.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316223404.102806-1-javierm@redhat.com