drm/i915: Disable snooping (userptr, set-cache-level) on gen4

The original gen4 has an issue where writes (both render and blt) into
snoopable pages are lost. We've previously worked around this in
userspace (ddx, igt) by simply not requesting snoopable buffers, but upon
rediscovering this problem for a third time, make the kernel reject such
requests with -ENODEV.

This disables snooping on userspace buffers for i965g and i965gm (original
gen4) machines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170906192424.26970-1-chris@chris-wilson.co.uk