svga: add DXGenMips command support

svga: add DXGenMips command support

For those formats that support hw mipmap generation, use the
DXGenMips command. Otherwise fallback to the mipmap generation utility.

Tested with piglit, OpenGL apps (Heaven, Turbine, Cinebench)

v2: make sure the texture surface was created with the render target bind flag
    set relocation flag to SVGA_RELOC_WRITE for the texture surface

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