mesa: use correct return statement for a void function

mesa: use correct return statement for a void function

Using return foo() is incorrect even if foo itself returns void.
Spotted by AppVeyor, as below:

teximage.c(3653) : warning C4098: 'copyteximage' : 'void' function returning a value

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>