vc4: Use u_box_pixels_to_blocks helper

Eliminates a ETC1 special case. In fact this unit conversion applies to
all formats; the original code path works since ETC1 is the only format
with blocks bigger than 1x1 supported by vc4 (I assume).

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Part-of: <!14370>
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