i965: Add a function to translate MESA_SHADER_* into DEBUG_* enums.

i965: Add a function to translate MESA_SHADER_* into DEBUG_* enums.

When compiling, we have a gl_shader_stage (MESA_SHADER_*) enum, and want
to know whether debugging is enabled for that stage.  This allows us to
easily translate it into the corresponding debug flag.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>