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 <326bbc75ae5821d6c8d1817b260a4d8d0fdf1f63@vmware.com>
Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>
Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler