panfrost: Move viewport desc emission out of panfrost_emit_for_draw()

panfrost: Move viewport desc emission out of panfrost_emit_for_draw()

Let's move the viewport descriptor emission logic to a dedicated helper
in order to shrink a bit the panfrost_emit_for_draw().

Note that this helper is placed in a new pan_cmdstream.c file where we
will group all cmdstream related helpers (everything that's related to
HW descriptor initialization emission).

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4083>