media: atomisp: Make atomisp_g_fmt_cap() default to YUV420

Make atomisp_g_fmt_cap() default to YUV420 so that it matches with what
atomisp_try_fmt_cap() and atomisp_queue_setup() do when they need to
pick a default pixelformat.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>