media: venus: vdec: update output buffer size during vdec_s_fmt()

Video driver maintains an internal context for the output buffer size.
During S_fmt() on capture plane, the output buffer size is not updated
in driver context. As a result, during buf_prepare(), the size of the
vb2_plane and internal size of the buffer, as maintained by the driver,
does not match. This leads to buf_prepare() failure.
Update the instance context for the output buffer size during s_fmt().

Signed-off-by: Mansur Alisha Shaik <mansur@codeaurora.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>