svga: remove assertions in svga_surface cast wrappers
svga: remove assertions in svga_surface cast wrappers

We don't do this for other cast wrappers.  And this will simplify some
code at call sites.

Reviewed-by: Neha Bhende <faec1a32a2d2624ef91f92ce8eed261ff4162c00@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler