mesa: Switch generate_mipmaps_compressed() to using TexImage2D to upload.

mesa: Switch generate_mipmaps_compressed() to using TexImage2D to upload.

The code was playing fast and loose with rowstrides, which meant that
if a driver chose anything different for its alignment requirements,
the generated mipmaps came out garbage.  Unlike the uncompressed case,
we can't generate mipmaps directly into image->Data, so by using
TexImage2D we cut out most of the weird logic that existed to generate
in-place into ->Data.  The up/downside is that the driver recovery
code for the fact that _mesa_generate_mipmaps whacked ->Data has to be
turned off for compressed now.

Fixes 6 piglit tests about compressed mipmap gen.