venus: vdec: fix format enumeration

find_format_by_index() stops enumerating formats as soon as the index
matches, and returns NULL if venus_helper_check_codec() finds out that
the format is not supported. This prevents formats to be properly
enumerated if a non-supported format is present, as the enumeration will
end with it.

Fix this by moving the call to venus_helper_check_codec() into the loop,
and keep enumerating when it fails.

Fixes: 29f0133e media: venus: use helper function to check supported codecs

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>