media: verisilicon: Fix crash when probing encoder

ctx->vpu_dst_fmt is no more initialized before calling hantro_try_fmt()
so assigne it to vpu_fmt led to crash the kernel.
Like for decoder case use 'fmt' as format for encoder and clean up
the code.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: db6f68b5 ("media: verisilicon: Do not set context src/dst formats in reset functions")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>