frontends/va/context: don't set max_references with num_render_targets

frontends/va/context: don't set max_references with num_render_targets

For HEVC and VP9 template's max_references are tied to the number of
surfaces associated with context. Later, the decoder is created if
max_references is different to zero.

But vaCreateContext() doesn't really need an array of VASurfaceIDs (see
https://lists.01.org/pipermail/intel-vaapi-media/2017-July/000052.html and
https://github.com/intel/libva/issues/251).

This patch removes the validation of the max_references at decoder
creation and also remove the assignation of num_render_targets to
max_references.

Signed-off-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7949>