drm/i915/gem: Single page objects are naturally contiguous

Small objects that only occupy a single page are naturally contiguous,
so mark them as such and allow them the special abilities that come with
it.

A more thorough treatment would extend i915_gem_object_pin_map() to
support discontiguous lmem objects, following the example of
ioremap_prot() and use get_vm_area() + remap_io_sg().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200101220736.1073007-1-chris@chris-wilson.co.uk