st/mesa: simplify st_generate_mipmap()

st/mesa: simplify st_generate_mipmap()

The whole st_generate_mipmap() function was overly complicated.  Now
we just call the new _mesa_prepare_mipmap_levels() function to prepare
the texture mipmap memory, then call the generate function which fills
in the texture images.

This fixes a failed assertion in llvmpipe/softpipe which is hit with the
new piglit generatemipmap-base-change test.  Also fixes some device errors
(format mismatches) with the VMware svga driver.

v2: fix a comment typo, per Sinclair

Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>