Build etnaviv on non-ARM architectures

I wanted to test-compile etnaviv on x86 after making a tree-wide change
to it.  Unfortunately, Kconfig has a bad dependency, so I couldn't.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>