mesa/main: make the CONSERVATIVE_RASTERIZATION_NV checks consistent

This just makes the logic of the checks for this enum the same for
gl{Enable,Disable} and for glIsEnabled. They are already functionally
the same, so this is just a minor code-cleanup.

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

@@ -1774,7 +1774,8 @@ _mesa_IsEnabled( GLenum cap )
          return ctx->IntelConservativeRasterization;
 
       case GL_CONSERVATIVE_RASTERIZATION_NV:
-         CHECK_EXTENSION(NV_conservative_raster);
+         if (!_mesa_has_NV_conservative_raster(ctx))
+            goto invalid_enum_error;
          return ctx->ConservativeRasterization;
 
       case GL_TILE_RASTER_ORDER_FIXED_MESA: