nvk: avoid casting between different format enums

What does this MR do and why?

nvk: avoid casting between different format enums

nil_image_init_info.format is a pipe_format and we were passing in a VkFormat.

Casting VK_FORMAT_R16_UINT to pipe_format makes it a PIPE_FORMAT_A8B8G8R8_SSCALED.

Convert the format with vk_format_to_pipe_format to get PIPE_FORMAT_R16_UINT instead

If this change is correct then it can be a fixup for 283dbce5