iris: fix fallback to swrast driver
The helper we use to query the kernel returns -1 if the getparam is not supported. Signed-off-by:Lionel Landwerlin <lionel.g.landwerlin@intel.com> Fixes: f402b7c5 ("iris: fail screen creation when kernel support is not there") Closes: mesa/mesa#3188 Reviewed-by:
Marcin Ślusarz <marcin.slusarz@intel.com> Part-of: <mesa/mesa!5838> (cherry picked from commit e3ddba73)