Since headless overrides create_mem, it needs to override finish_create
too. Fixes a segfault in nvk that was caused by us mixing
wsi_create_null_image_mem with wsi_finish_create_blit_context, which
would then call CmdCopyImageToBuffer with image->blit.buffer == NULL
Fixes a cts failure on nvk in:
dEQP-VK.image.swapchain_mutable.headless.2d.r8g8b8a8_unorm_b8g8r8a8_unorm_clear_copy_format_list
and several others
Fixes: 579578f10a ("vulkan/wsi/drm: Break create_prime_image in pieces")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34646>