pan/midgard: Lower txd with lower_tex

pan/midgard: Lower txd with lower_tex

This is a hack since we do have native gradient stuff, but for the
moment I'm more interested in conformance and the lowered code is good
enough. Fixes
dEQP-GLES3.functional.shaders.texture_functions.texturegrad.sampler2d_fixed_fragment

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3169>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3169>