frontends/va: Flush after unmapping VAImageBufferType

frontends/va: Flush after unmapping VAImageBufferType

If application changed image data we need to flush on unmap to make the
changes visible. This will also flush if the mapping was used only for
reading, but we can't know that as vaMapBuffer doesn't have a parameter
to specify if read or write is requested.

Cc: mesa-stable
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9774

Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25102>