panfrost: Add a knob to disable the UBO -> push constants optimization

panfrost: Add a knob to disable the UBO -> push constants optimization

I'm just too lazy to implement the logic to prepare push constant
buffers in the Vulkan driver. Besides, Vulkan has explicit push
constants, which AFAIK is not handled in the compiler backends yet,
and that will probably conflict with the UBO -> push constant
promotion.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9517>