i965: avoid unnecessary calls to brw_wm_is_glsl()

i965: avoid unnecessary calls to brw_wm_is_glsl()

This function scans the shader to see if it has any GLSL features like
conditionals and loops.  Calling this during state validation is expensive.
Just call it when the shader is given to the driver and save the result.

There's some new/temporary assertions to be sure we don't get out of sync
on this.