panfrost: Move panfrost_attach_vt_framebuffer() to pan_cmdstream.c

panfrost: Move panfrost_attach_vt_framebuffer() to pan_cmdstream.c

Move panfrost_attach_vt_framebuffer() to pan_cmdstream.c and change its
name to panfrost_vt_attach_framebuffer() so we can use a consistent
prefix (panfrost_vt_) for all helpers initializing/updating
midgard_payload_vertex_tiler fields.

Note that the function only initializes one VT object now.

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>