drm/etnaviv: fix get pages error path in etnaviv_gem_vaddr

In case that etnaviv_gem_get_pages is unable to get the required
pages the object mutex needs to be unlocked. Also return NULL in
this case instead of propagating the error, as callers of this
function might not be prepared to handle a pointer error, but
expect this call to follow the semantics of a plain vmap to return
NULL in case of an error.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>