drm/i915/gem: Prevent NULL pointer dereference on missing ctx->vm

If the ctx->vm is freed before we can acquire a local reference to it,
we proceed to call i915_vm_put(NULL), which is invalid.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: 5dbd2b7b ("drm/i915/gem: Convert vm idr to xarray")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123152602.1432282-1-chris@chris-wilson.co.uk