svga: move svga_texture() casts/calls in svga_surface_copy()

svga: move svga_texture() casts/calls in svga_surface_copy()

To fix failed assertions when calling glCopyBufferSubData().

svga_texture() asserts that the resource is a texture.  Simply move the
calls to svga_texture() after the code that handles non-texture copies
so that we don't call it with non-texture resources.

Fixes glean bufferObject failure.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>