turnip: rework format table to support r5g5b5a1_unorm/b5g5r5a1_unorm

turnip: rework format table to support r5g5b5a1_unorm/b5g5r5a1_unorm

These formats are an exception that can't be modeled in the current format
table. Switch to a table with only a single a6xx_format per vk format,
and deal with the exceptions separately (currently the only exception is
10_10_10_2_UNORM which has a different color format).

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3806>