panfrost: Use txl instead of tex in the blitter

panfrost: Use txl instead of tex in the blitter

We always blit from a particular level, so it's a waste to compute the LOD.
This corresponds to a simple texture instruction with implement 0 LOD, which is
the optimal texturing path on Bifrost -- it maps to TEXS_2D but does not require
helper invocations.

Functional change on Bifrost: Blit shaders no longer set .computed_lod or
shader_contains_barrier.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15123>