i965: Add norbc debug option

This INTEL_DEBUG option disables lossless compression (also known
as render buffer compression).

v2: (Matt) Use likely(!lossless_compression_disabled) instead of
           !likely(lossless_compression_disabled)
    (Grazvydas) Update docs/envvars.html

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/envvars.html

@@ -166,6 +166,7 @@ See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
    <li>vec4 - force vec4 mode in vertex shader</li>
    <li>spill_fs - force spilling of all registers in the scalar backend (useful to debug spilling code)</li>
    <li>spill_vec4 - force spilling of all registers in the vec4 backend (useful to debug spilling code)</li>
+   <li>norbc - disable single sampled render buffer compression</li>
 </ul>
 </ul>