mesa/main: condition GL_DEPTH_STENCIL on ARB_depth_texture

mesa/main: condition GL_DEPTH_STENCIL on ARB_depth_texture

EXT_packed_depth_stencil is supported by all drivers, but
ARB_depth_texture isn't (notably nouveau_vieux). This should avoid
passing unexpected values down to ChooseTextureFormat.

The EXT_packed_depth_stencil spec does not make any explicit references
to requiring ARB_depth_texture in order to allow textures with that
format, however if there is no dependency, ARB_depth_texture would be
practically implied by the extension.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "10.0 10.1" <mesa-stable@lists.freedesktop.org>
Note for 10.0 backport: This will produce a conflict, the solution is to
move the surrounding if as well.