drm/i915: Use ttm mmap handling for ttm bo's.

Use the ttm handlers for servicing page faults, and vm_access.

We do our own validation of read-only access, otherwise use the
ttm handlers as much as possible.

Because the ttm handlers expect the vma_node at vma->base, we slightly
need to massage the mmap handlers to look at vma_node->driver_private
to fetch the bo, if it's NULL, we assume i915's normal mmap_offset uapi
is used.

This is the easiest way to achieve compatibility without changing ttm's
semantics.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610070152.572423-5-thomas.hellstrom@linux.intel.com

drm/i915/selftests: Wrap vm_mmap() around GEM objects

Provide a utility function to create a vma corresponding to an mmap() of
our device. And use it to exercise the equivalent of userspace
performing a GTT mmap of our objects.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107180601.30815-4-chris@chris-wilson.co.uk