meta: Don't save and restore fog state when there is no fog state

meta: Don't save and restore fog state when there is no fog state

I wonder if the better solution is to have _mesa_meta_GenerateMipmap not
use MESA_META_ALL for the GLSL path.  Even on compatibility profiles
there is no reason to save and restore fog on this path.

NOTE: This is a candidate for the 9.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Lu Hua <huax.lu@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54295