drm/i915/selftests: fix check for intel IOMMU

An earlier bugfix tried to work around this build failure:

drivers/gpu/drm/i915/selftests/mock_gem_device.c: In function 'mock_gem_device':
drivers/gpu/drm/i915/selftests/mock_gem_device.c:151:20: error: 'struct dev_archdata' has no member named 'iommu'

Checking for CONFIG_IOMMU_API is not sufficient as a compile-time
test since that may be enabled in configurations that have neither
INTEL_IOMMU not AMD_IOMMU enabled. This changes the check to
INTEL_IOMMU instead, as this is the only case we actually care about.

Fixes: f46f156e ("drm/i915/selftests: Only touch archdata.iommu when it exists")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171005120749.400818-1-arnd@arndb.de