media: verisilicon: Do not change context bit depth before validating the format

It is needed to check if the proposed pixels format is valid before
updating context bit depth and other internal states.
Stop using ctx->bit_depth to check format depth match and return
result to the caller.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>