mesa: Enable svga for x86 only

Enable svga only on x86/x86_64 since some arches e.g. riscv64 do not
support it

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>