anv/formats: Update the three-channel BC1 mappings

The procedure for decompressing an opaque BC1 Vulkan format is dependant on the
comparison of two colors stored in the first 32 bits of the compressed block.
Here's the specified OpenGL (and Vulkan) behavior for reference:

   The RGB color for a texel at location (x,y) in the block is given by:

      RGB0,              if color0 > color1 and code(x,y) == 0
      RGB1,              if color0 > color1 and code(x,y) == 1
      (2*RGB0+RGB1)/3,   if color0 > color1 and code(x,y) == 2
      (RGB0+2*RGB1)/3,   if color0 > color1 and code(x,y) == 3

      RGB0,              if color0 <= color1 and code(x,y) == 0
      RGB1,              if color0 <= color1 and code(x,y) == 1
      (RGB0+RGB1)/2,     if color0 <= color1 and code(x,y) == 2
      BLACK,             if color0 <= color1 and code(x,y) == 3

The sampling operation performed on an opaque DXT1 Intel format essentially
hard-codes the comparison result of the two colors as color0 > color1. This
means that the behavior is incompatible with OpenGL and Vulkan. This is stated
in the SKL PRM, Vol 5: Memory Views:

   Opaque Textures (DXT1_RGB)
      Texture format DXT1_RGB is identical to DXT1, with the exception that the
      One-bit Alpha encoding is removed. Color 0 and Color 1 are not compared, and
      the resulting texel color is derived strictly from the Opaque Color Encoding.
      The alpha channel defaults to 1.0.

      Programming Note
      Context: Opaque Textures (DXT1_RGB)
      The behavior of this format is not compliant with the OGL spec.

The opaque and non-opaque BC1 Vulkan formats are specified to be decoded in
exactly the same way except the BLACK value must have a transparent alpha
channel in the latter. Use the four-channel BC1 Intel formats with the alpha
set to 1 to provide the behavior required by the spec.

v2 (Kenneth Graunke):
- Provide a more detailed commit message.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100925
Cc: <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
Signed-off-by: Nanley Chery <[email protected]>

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