mesa: Let compute shaders work in compatibility profiles

The extension is already advertised in compatibility profile, but
the _mesa_has_compute_shaders only returns true in core profile.
If we advertise it, we should allow it to work.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>

src/mesa/main/context.h

 static inline bool
 _mesa_has_compute_shaders(const struct gl_context *ctx)
 {
-   return (ctx->API == API_OPENGL_CORE && ctx->Extensions.ARB_compute_shader) ||
+   return _mesa_has_ARB_compute_shader(ctx) ||
       (ctx->API == API_OPENGLES2 && ctx->Version >= 31);
 }