mesa: Don't assert on the compressed convertformat for GenerateMipmaps.

mesa: Don't assert on the compressed convertformat for GenerateMipmaps.

This assertion doesn't make any sense to me -- the convertFormat is
already something valid (tested above), and the BaseFormat dictated by
convertFormat doesn't matter to the function about to be called (it's
the datatype/comps that were pulled out of convertFormat).

Fixes assertion failure in
GL_EXT_texture_compression_rgtc/fbo-generatemipmap-formats

(still has a rendering failure in NPOT like S3TC does).

Reviewed-by: Brian Paul <brianp@vmware.com>