mesa/cs: Add a MESA_SHADER_COMPUTE stage and update switch statements.

mesa/cs: Add a MESA_SHADER_COMPUTE stage and update switch statements.

This patch adds MESA_SHADER_COMPUTE to the gl_shader_stage enum.
Also, where it is trivial to do so, it adds a compute shader case to
switch statements that switch based on the type of shader.  This
avoids "unhandled switch case" compiler warnings.

Reviewed-by: Matt Turner <mattst88@gmail.com>