anv: Rename cpp variable to "bs"

anv: Rename cpp variable to "bs"

cpp (chars-per-pixel) is an integer that fails to give useful data
about most compressed formats. Instead, rename it to "bs" which
stands for block size (in bytes).

v2: Rename vk_format_for_bs to vk_format_for_size (Chad)
    Use "block size" instead of "bs" in error message (Chad)

Reviewed-by: Chad Versace <chad.versace@intel.com>