mesa/st: support lowering multi-planar YUV

Support multi-planar YUV for external EGLImage's (currently just in the
dma-buf import path) by lowering to multiple texture fetch's for each
plane and CSC in shader.

There was some discussion of alternative approaches for tracking the
additional UV or U/V planes:

  https://lists.freedesktop.org/archives/mesa-dev/2016-September/127832.html

They all seemed worse than pipe_resource::next

Signed-off-by: Rob Clark <robdclark@gmail.com>

src/mesa/state_tracker/st_atom_shader.c

@@ -51,6 +51,7 @@
 #include "st_context.h"
 #include "st_atom.h"
 #include "st_program.h"
+#include "st_texture.h"
 
 
 /** Compress the fog function enums into a 2-bit value */
@@ -142,6 +143,8 @@ update_fp( struct st_context *st )
       }
    }
 
+   key.external = st_get_external_sampler_key(st, &stfp->Base.Base);
+
    st->fp_variant = st_get_fp_variant(st, stfp, &key);
 
    st_reference_fragprog(st, &st->fp, stfp);