media: mtk-vcodec: Always signal source change event on format change

Currently the driver signals the source change event only in case of
a midstream resolution change, however the initial format detection
is also defined as a source change by the V4L2 codec API specification.
Fix this by signaling the event after the initial header is parsed as
well.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Wu-Cheng Li <wuchengli@chromium.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>