Fast-clears of GL_ALPHA16 textures are broken on TGL
convert_fast_clear_color expects that the given clear color channels are not swizzled with respect to the given pipe format. iris_clear_texture however, gives it a clear color that has been swizzled according to an ISL format. This results in a corrupted clear color.
For example, consider PIPE_FORMAT_A16_UNORM-formatted resources. The clear pixel is unpacked as (R, 0, 0, 0) [instead of (0, 0, 0, A)] because the ISL format used is ISL_FORMAT_R16_UNORM. When convert_fast_clear_color goes to fill the R, G, and B channels with zeroes, it actually discards the alpha value that iris_clear_texture stored in the R channel. The resulting clear color is (0, 0, 0, 0).