st/va: delay decoder creation until max_references is known

st/va: delay decoder creation until max_references is known

In general max_references cannot be based on num_render_targets.

This patch allows to allocate buffers with an accurate size.
I.e. no more than necessary. For other codecs it is a fixed
value 2.

This is similar behaviour as vaapi/vdpau-driver.

For now HEVC case defaults to num_render_targets as before.
But it could also benefits this change by setting a more
accurate max_references number in handlePictureParameterBuffer.

Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>