intel: Unconditionally enable support for S8_Z24 texture format

intel: Unconditionally enable support for S8_Z24 texture format

Commit b5c847c7ca06823af3b72324056a2e478caca70b erroneously disabled
support for S8_Z24 texture format when the context required separate
stencil (intel_context.must_use_separate_stencil).

But the GL spec requires implementations to support GL_DEPTH24_STENCIL8.
So we better find a way to fake it...

From page 180 (196 of pdf) of the OpenGL 3.0 spec:
    In addition, implementations are required to support the following
    sized internal [texture] formats.

    [...]

    - Combined depth+stencil formats: DEPTH32F_STENCIL8 and and
      DEPTH24_STENCIL8.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad@chad-versace.us>