panfrost: Fix 32-bit warning for `indices`

panfrost: Fix 32-bit warning for `indices`

../src/gallium/drivers/panfrost/pan_context.c: In function ‘panfrost_draw_vbo’:
../src/gallium/drivers/panfrost/pan_context.c:1551:70: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
                 ctx->payloads[PIPE_SHADER_FRAGMENT].prefix.indices = (u64) NULL;
                                                                      ^

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reported-by: Icecream95 <ixn@keemail.me>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3543>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3543>