agx: Implement nir_texop_txf_ms

Mutlisampled texture fetch (txf_ms) is encoded like regular txf. However, we now
need to pack the multisample index in the right place, which we do by extending
our existing NIR texture source lowering pass. 2D MS arrays use a new value of
dim which requires tweaking the encoding slightly. Otherwise, everything is
bog standard.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19871>

src/asahi/compiler/agx_compiler.h

@@ -316,7 +316,7 @@ typedef struct {
    bool invert_cond : 1;
 
    /* TODO: Handle tex ops more efficient */
-   enum agx_dim dim : 3;
+   enum agx_dim dim : 4;
    bool offset : 1;
    bool shadow : 1;