meta: Unconditionally set GL_SKIP_DECODE_EXT

meta: Unconditionally set GL_SKIP_DECODE_EXT

The path that depends on this will be avoided (by fallback_required) if
the extension is not supported.  _mesa_set_sampler_srgb_decode does not
generate GL errors (by design), so there are no problems there.

I kept this change separate and last because it is one of the few in the
series that is not a candidate for the stable branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>