drm/i915: Discard a misplaced GGTT vma

Across the many users of the GGTT vma (internal objects, mmapings,
display etc), we may end up with conflicting requirements for the
placement. Currently, we try to resolve the conflict by unbinding the
vma and rebinding it to match the new constraints; over time we will end
up with a GGTT that matches the most strict constraints over all
concurrent users. However, this causes a problem if the vma is currently
in use as we must wait until it is idle before moving it. But there is
no restriction on the number of views we may use (apart from the limited
size of the GGTT itself), and so if the active vma does not meet our
requirements, try and build a new one!

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/20200605165258.1483-1-chris@chris-wilson.co.uk