panfrost: Delay payloads[].offset_start initialization

panfrost: Delay payloads[].offset_start initialization

panfrost_draw_vbo() Might call the primeconvert/without_prim_restart
helpers which will enter the ->draw_vbo() again. Let's delay
payloads[].offset_start initialization so we don't initialize them
twice.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>