freedreno/a6xx: Use fd6_pipe2tex() for the 2D src format.

freedreno/a6xx: Use fd6_pipe2tex() for the 2D src format.

The differences between the two are:

- tex supports more formats (fine, we've already decided on which formats
  we can blit)
- FMT6_10_10_10_2_UNORM instead of FMT6_10_10_10_2_UNORM_DEST.
- FMT6_A8_UNORM for PIPE_FORMAT_A8_UNORM.

For 1555 support we get another difference between tex and color, and we
need 2/3 of the tex differences for blit srcs, so switch it over.  This
also matches what turnip does (except it doesn't have A8 as a format).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13046>