assembler: Adopt brw_structs.h from mesa

assembler: Adopt brw_structs.h from mesa

Finally merge both brw_structs.h from mesa. One detail has risen in that
last commit, the msg_control field of data port message descriptors.

Mesa's msg_control field is sometimes split with messages-specific
fields where the assembler (at least for recent generations) exposes the
full msg_control value in the send instruction.

As libva's shaders encodes the full msg_control value in its send
instructions, I've chosen to not take the split msg_control from mesa.
It's absolutely possible to have a patch fixing that divergence at some
later point.

I've also kept a hack introduced with ironlake to not have to rewrite
shaders (that encode msg_control in the text, remember), and thus
creates a another difference with Mesa.

-	 GLuint msg_control:3;
-	 GLuint msg_type:3;
+	 GLuint msg_control:4;
+	 GLuint msg_type:2;

Once again, I've made sure that re-generating libva's shaders don't show
any difference.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>