panfrost: Pass a memory pool to pan_blit_ctx_init()

panfrost: Pass a memory pool to pan_blit_ctx_init()

Pass a memory pool to pan_blit_ctx_init() instead of creating a new pool.
Useful for Vulkan since the descriptor pool is at the command buffer
level and is thus shared by all blit batches. Doing this will save us a
BO ownership transfer.

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/11695>