mesa: fix error checking of DXT sRGB formats in _mesa_base_tex_format()

mesa: fix error checking of DXT sRGB formats in _mesa_base_tex_format()

For formats such as GL_COMPRESSED_SRGB_S3TC_DXT1_EXT we need to
have both the GL_EXT_texture_sRGB and GL_EXT_texture_compression_s3tc
extensions.  This patch adds the missing check for the later.

Found when checking out https://bugs.freedesktop.org/show_bug.cgi?id=65173

NOTE: This is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>