panfrost: Combine get_index_buffer with bound computation

panfrost: Combine get_index_buffer with bound computation

These operations are intertwined since there are optimizations that will
want to "double dip". In particular for user index buffers we'd want to
upload simultaneous with index computation. For resources we'd like to
keep resource related code together.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3880>