mesa/main: make the CONSERVATIVE_RASTERIZATION_INTEL checks consistent

INTEL_conservative_rasterization isn't exposed on compatibility
contexts, nor for GLES 3.0 and below. We already do this correctly for
gl{Enable,Disable}, but we should do the same for glIsEnabled as well.
Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/enable.c

@@ -1770,7 +1770,8 @@ _mesa_IsEnabled( GLenum cap )
          return ctx->Color.BlendCoherent;
 
       case GL_CONSERVATIVE_RASTERIZATION_INTEL:
-         CHECK_EXTENSION(INTEL_conservative_rasterization);
+         if (!_mesa_has_INTEL_conservative_rasterization(ctx))
+            goto invalid_enum_error;
          return ctx->IntelConservativeRasterization;
 
       case GL_CONSERVATIVE_RASTERIZATION_NV: