i965: Add support for GL_EXT_packed_float.

This doesn't cover support for this format as a renderbuffer yet.  The
spec allows implementations to not support it, though it is something
we do want to support.

Only one failure in piglit on gen6, which is texwrap with bordercolor
(as usual).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/relnotes-7.12.html

@@ -37,6 +37,7 @@ tbd
 <h2>New features</h2>
 <ul>
 <li>GL_EXT_texture_shared_exponent (i965)
+<li>GL_EXT_packed_float (i965)
 </ul>