drm/i915: Rename i915_gem_context_get_vm_rcu to i915_gem_context_get_eb_vm

The important part isn't so much that this does an rcu lookup - that's
more an implementation detail, which will also be removed.

The thing that makes this different from other functions is that it's
gettting you the vm that batchbuffers will run in for that gem
context, which is either a full ppgtt stored in gem->ctx, or the ggtt.

We'll make more use of this function later on.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210902142057.929669-5-daniel.vetter@ffwll.ch