drm/i915/selftests: Mark the whole mock device as DMA capable

Being a mock device, we suffer no DMA restrictions, so set the coherent
mask to 64b.

v2: Fix up mock_huge_selftests

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109243
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107181856.23789-1-chris@chris-wilson.co.uk