svga: add new function svga_clear_texture()

svga: add new function svga_clear_texture()

To clear texture this function can be used. This is part of
ARB_clear_texture extension. Basically this extension allows you to
clear texture with given color values.

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